South Korea has fired shells into North Korean waters after their rivals sent more than 100 rounds below the disputed sea boundary during a live-fire drill. The heated artillery exchange was . prompted by an unusual warning fax sent from the North to the South, . informing them of their live-fire drill. Though . it is not unusual for the reclusive nation to practice at sea, South . Korean officials were alarmed by the alert, which they claim 'indicates . their hostile intention'. It comes just days after Kim Jong Un's reclusive nation threatened to launch a 'new form' of nuclear power tests - and coincidentally during the annual South Korea-US military practice, which North Korea dubs a rehearsal for invasion. Scroll down for video . Dispute: The South Korean military shot shells into North Korean waters during a drill after their rivals fired across the sea boundary during their own practice . Residents on the South Korean island of Yeonpyeong, metres south of the border, were told to seek shelter during the drill. There were no reports of injuries . The heated exchange took place during the US Marines' annual military training with South Korea, which the North brands rehearsal for invasion . According to South Korean military officials, the North fired more than 500 rounds during a three-hour drill - 100 of which crossed the border. The South fired back 300 rounds from its K-9 self-propelled howitzer batteries, and scrambled F-15 jets. Residents on the South Korean island of Yeonpyeong, located in the battle-stricken Yellow Sea, were evacuated to shelters during the exchange. The Northern Limit Line, a maritime border that wraps itself round a part of the North's coastline, has been the scene of frequent clashes. In 2010, two civilians and two marines on the island were killed by North Korean military during a border dispute. The North claimed it was provoked by South Korean military exercises. Earlier that year, 46 people died off the coast of nearby Baengnyeong island after a warship sank. South Korea claims it was gunned down by the North, which their rival neighbour denies. 'The fact that they have sent such a message to us indicates their hostile intention.' said Wi Yong-Seop, a spokesman for the South Korean Defence Ministry. 'The aim is to threaten us and rack up tension on the Yellow Sea border and the overall Korean Peninsula,' he said. South Korean military (pictured) was alarmed by an early-morning fax from the North warning them to control their vessels in the region as they would be 'training'. Though live-fire training is not uncommon, it is unusual for the rivals to warn one another. The South said it showed hostile intentions . Training: Amphibious assault vehicles of the South Korean Marine Corps were throwing smoke bombs in a drill with the US where they practised moving from sea to shore. But during the morning, more than 100 shells from the North crossed the border. The South responded by shooting back 300 rounds . The North calls the South Korea-US drills a rehearsal for invasion despite the allies claiming they are routine and defensive . Disputed border: Yeonpyeong Island has been subjected to numerous bloody naval skirmishes over the years as North and South Koreans train near the border . The message, faxed to South Korea's . Second Fleet headquarters at 8am local time, warned officials to control . their vessels in the tensely-guarded western sea as they would be . conducting live-fire drills later in the day, officials said. It stated that all action would be north of the Northern Limit Line (NLL), but did not specify what time it would take place. Seoul officials claim the move is an . expression of Pyongyang's frustration at failing to win outside aid in . recent weeks despite numerous attempts. And . since the UN published a report early this month accusing Kim Jong-Un . of war crimes akin to Nazi Germany, the reclusive nation has increased . threatening rhetoric and conducted a series of rocket and ballistic . missile launches. This weekend, tensions rapidly intensified. On Saturday, North Korea also accused the South of 'gangster-like' behaviour by 'abducting' one of its fishing boats and threatened to retaliate. The South said it had sent the boat back after it drifted into its waters. Hours later, Pyongyang threatened to . conduct a fourth nuclear test at some point. After the North's warning fax early this morning, South Korea responded that it would react strongly if provoked. North Korea blasted the response, with one military official Yun Jong Bum telling the North's official news agency: 'The boneheads appear to have completely forgotten the fact that Yeonpyeong island was smashed by our military's bolt of lightning a few years ago.' There were no specific targets during this morning's exchange, and no . shells from either side were fired at any land or military . installations, claimed an official with South Korea's Joint Chiefs of Staff. The Northern Limit Line was drawn up at the end of the . 1950-53 Korean War - and North Korea does not recognise it. Five South Korean border islands were evacuated as the Yellow Sea became engulfed by smoke during the drills at noon local time . Last time the US trained with South Korea, the North sent near-daily threats, including warnings of nuclear strikes against Seoul and Washington . The Northern Limit Line was drawn up at the end of the 1950-53 Korean War - and North Korea does not recognise it . The two sides . are still technically at war as the conflict ended in a mere truce, not a . treaty. 'It's up to the two . militaries either to recognise or reject their own claimed line, and . challenge the other's - this goes back and forth, so this is probably . another episode of that,' said Daniel Pinkston of the International . Crisis Group. Earlier in . 2010, a South Korean naval vessel was sunk close to the line by what an . international commission said was a North Korean torpedo, although the . North denies involvement. Today, Kang . Myeong-sung, a resident speaking from a shelter on Yeonpyeong island, which is in . sight of North Korean territory, said that anxious islanders were . huddled together in shelters. Kang said he didn't see any fighter jets, . but he could hear the boom of artillery fire. It . is not the first time conflict has broken out during the South Korea-US . military drills, which the North calls a rehearsal for invasion. The . allies say they're routine and defensive. Last spring, tension spiked after a . near-daily barrage of North Korean threats, including warnings of . nuclear strikes against Seoul and Washington, following international . criticism of Pyongyang's third nuclear test in February of that year. Pyongyang routinely test-fires artillery and missiles into the ocean, but it's rare for the country to disclose such training plans in advance . On Sunday, Pyongyang threatened to conduct a fourth nuclear test at some point, though Seoul says there are no signs of an imminent detonation . But the North has gradually dialed . down its threats and sought to improve ties with South Korea in what . foreign analysts say is an attempt to lure international investment and . aid. There has been no . major breakthrough in the North's reported push to win outside aid, . however, with Washington and Seoul calling on the North to first take . disarmament steps to prove its sincerity about improving ties, analysts . say. The North Korean live-fire drills and the country's hints at a nuclear test are meant to express anger and frustration over what the North sees as little improvement in progress in its ties with South Korea and the US, said Lim Eul Chul, a North Korea expert at South Korea's Kyungnam University. Lim said the North might conduct a fourth nuclear test and launch other provocations to try to wrest the outside concessions it wants. The Korean Peninsula remains in a technical state of war because the 1950-53 Korean War ended with an armistice, not a peace treaty. About 28,500 American troops are deployed in South Korea to deter potential aggression from North Korea.
North Korea conducted live-fire drill at 12.15 (3.15am GMT), firing into disputed waters near a South Korean island .
Reclusive nation sent unusual 'warning fax' to South Korea before military practice, which alarmed officials .
When shots crossed border, South Korean military responded by firing back 300 rounds and scrambling F-15 jets .
Heated exchange happened during South's annual training with US, which North brands 'rehearsal for invasion'
Residents of five border islands evacuated to shelters, in 2010 four islanders were killed by North Korea military .
Test-fire drills are normal for North Korea, but they usually don't issue warning ahead of time .
South Korea has fired shells into North Korean waters after their rivals sent more than 100 rounds below the disputed sea boundary during a live-fire drill. The heated artillery exchange was . prompted by an unusual warning fax sent from the North to the South, . informing them of their live-fire drill. Though . it is not unusual for the reclusive nation to practice at sea, South . Korean officials were alarmed by the alert, which they claim 'indicates . their hostile intention'. It comes just days after Kim Jong Un's reclusive nation threatened to launch a 'new form' of nuclear power tests - and coincidentally during the annual South Korea-US military practice, which North Korea dubs a rehearsal for invasion. Scroll down for video . Dispute: The South Korean military shot shells into North Korean waters during a drill after their rivals fired across the sea boundary during their own practice . Residents on the South Korean island of Yeonpyeong, metres south of the border, were told to seek shelter during the drill. There were no reports of injuries . The heated exchange took place during the US Marines' annual military training with South Korea, which the North brands rehearsal for invasion . According to South Korean military officials, the North fired more than 500 rounds during a three-hour drill - 100 of which crossed the border. The South fired back 300 rounds from its K-9 self-propelled howitzer batteries, and scrambled F-15 jets. Residents on the South Korean island of Yeonpyeong, located in the battle-stricken Yellow Sea, were evacuated to shelters during the exchange. The Northern Limit Line, a maritime border that wraps itself round a part of the North's coastline, has been the scene of frequent clashes. In 2010, two civilians and two marines on the island were killed by North Korean military during a border dispute. The North claimed it was provoked by South Korean military exercises. Earlier that year, 46 people died off the coast of nearby Baengnyeong island after a warship sank. South Korea claims it was gunned down by the North, which their rival neighbour denies. 'The fact that they have sent such a message to us indicates their hostile intention.' said Wi Yong-Seop, a spokesman for the South Korean Defence Ministry. 'The aim is to threaten us and rack up tension on the Yellow Sea border and the overall Korean Peninsula,' he said. South Korean military (pictured) was alarmed by an early-morning fax from the North warning them to control their vessels in the region as they would be 'training'. Though live-fire training is not uncommon, it is unusual for the rivals to warn one another. The South said it showed hostile intentions . Training: Amphibious assault vehicles of the South Korean Marine Corps were throwing smoke bombs in a drill with the US where they practised moving from sea to shore. But during the morning, more than 100 shells from the North crossed the border. The South responded by shooting back 300 rounds . The North calls the South Korea-US drills a rehearsal for invasion despite the allies claiming they are routine and defensive . Disputed border: Yeonpyeong Island has been subjected to numerous bloody naval skirmishes over the years as North and South Koreans train near the border . The message, faxed to South Korea's . Second Fleet headquarters at 8am local time, warned officials to control . their vessels in the tensely-guarded western sea as they would be . conducting live-fire drills later in the day, officials said. It stated that all action would be north of the Northern Limit Line (NLL), but did not specify what time it would take place. Seoul officials claim the move is an . expression of Pyongyang's frustration at failing to win outside aid in . recent weeks despite numerous attempts. And . since the UN published a report early this month accusing Kim Jong-Un . of war crimes akin to Nazi Germany, the reclusive nation has increased . threatening rhetoric and conducted a series of rocket and ballistic . missile launches. This weekend, tensions rapidly intensified. On Saturday, North Korea also accused the South of 'gangster-like' behaviour by 'abducting' one of its fishing boats and threatened to retaliate. The South said it had sent the boat back after it drifted into its waters. Hours later, Pyongyang threatened to . conduct a fourth nuclear test at some point. After the North's warning fax early this morning, South Korea responded that it would react strongly if provoked. North Korea blasted the response, with one military official Yun Jong Bum telling the North's official news agency: 'The boneheads appear to have completely forgotten the fact that Yeonpyeong island was smashed by our military's bolt of lightning a few years ago.' There were no specific targets during this morning's exchange, and no . shells from either side were fired at any land or military . installations, claimed an official with South Korea's Joint Chiefs of Staff. The Northern Limit Line was drawn up at the end of the . 1950-53 Korean War - and North Korea does not recognise it. Five South Korean border islands were evacuated as the Yellow Sea became engulfed by smoke during the drills at noon local time . Last time the US trained with South Korea, the North sent near-daily threats, including warnings of nuclear strikes against Seoul and Washington . The Northern Limit Line was drawn up at the end of the 1950-53 Korean War - and North Korea does not recognise it . The two sides . are still technically at war as the conflict ended in a mere truce, not a . treaty. 'It's up to the two . militaries either to recognise or reject their own claimed line, and . challenge the other's - this goes back and forth, so this is probably . another episode of that,' said Daniel Pinkston of the International . Crisis Group. Earlier in . 2010, a South Korean naval vessel was sunk close to the line by what an . international commission said was a North Korean torpedo, although the . North denies involvement. Today, Kang . Myeong-sung, a resident speaking from a shelter on Yeonpyeong island, which is in . sight of North Korean territory, said that anxious islanders were . huddled together in shelters. Kang said he didn't see any fighter jets, . but he could hear the boom of artillery fire. It . is not the first time conflict has broken out during the South Korea-US . military drills, which the North calls a rehearsal for invasion. The . allies say they're routine and defensive. Last spring, tension spiked after a . near-daily barrage of North Korean threats, including warnings of . nuclear strikes against Seoul and Washington, following international . criticism of Pyongyang's third nuclear test in February of that year. Pyongyang routinely test-fires artillery and missiles into the ocean, but it's rare for the country to disclose such training plans in advance . On Sunday, Pyongyang threatened to conduct a fourth nuclear test at some point, though Seoul says there are no signs of an imminent detonation . But the North has gradually dialed . down its threats and sought to improve ties with South Korea in what . foreign analysts say is an attempt to lure international investment and . aid. There has been no . major breakthrough in the North's reported push to win outside aid, . however, with Washington and Seoul calling on the North to first take . disarmament steps to prove its sincerity about improving ties, analysts . say. The North Korean live-fire drills and the country's hints at a nuclear test are meant to express anger and frustration over what the North sees as little improvement in progress in its ties with South Korea and the US, said Lim Eul Chul, a North Korea expert at South Korea's Kyungnam University. Lim said the North might conduct a fourth nuclear test and launch other provocations to try to wrest the outside concessions it wants. The Korean Peninsula remains in a technical state of war because the 1950-53 Korean War ended with an armistice, not a peace treaty. About 28,500 American troops are deployed in South Korea to deter potential aggression from North Korea.
live-fire drill. The heated artillery exchange was . prompted by an unusual warning fax sent South . Korean officials were alarmed by the military practice, which North Korea dubs a into North Korean waters during a drill after annual military training with South Korea, which the North brands rehearsal for invasion . According to South Korean military officials, the scrambled F-15 jets. Residents on the South Korean island of Yeonpyeong, located in the two marines on the island were killed by North South responded by shooting back 300 rounds . The it. Five South Korean border islands were time the US trained with South Korea, the North islanders were . huddled together in shelters.