By . Mail Foreign Service . PUBLISHED: . 07:40 EST, 27 July 2012 . | . UPDATED: . 20:15 EST, 27 July 2012 . Syrian government forces yesterday launched an attack on the country’s largest city in what Britain and the U.S. warned may become a massacre. Artillery fire and an air bombardment pounded rebel-held areas of Aleppo to prepare the ground for a full onslaught. The rebels have been gradually seizing parts of Syria’s most strategically-important city, and for the past week fighting fierce  battles with President Bashar al-Assad’s forces. Scroll down for video . Syrian rebels patrol on the outskirts of Aleppo yesterday. The U.S. haswarned of a massacre in the city, after President Bashar Assad's forces launched ground and air bombardments in preparation for a major onslaught . Unsafe: Syrian civilians leave their homes in Aleppo during a lull in fighting between opposition forces and army troops . Free Syrian Army fighters battle government security forces during the siege of the Shaar district police station in Aleppo, which was later overrun by the rebel forces yesterday . But the pro-government al-Watan . newspaper warned that the ‘mother of all battles’ was about to start as . Assad’s regime tries to recapture all of the city. The battle for . Aleppo, which is home to 2.5million, is being seen as a turning point in . the 16-month uprising against Assad in a conflict where both the rebels . and the government have struggled to gain the upper hand. The US State Department said it had . received credible reports of tank columns moving on Aleppo, along with . air strikes by helicopter gunships. These represented a serious . escalation of Assad’s efforts to crush his opponents. Taken from a video uploaded to YouTube, this image purports to show members of Assad forces being held prisoner by rebels in Allepo . Innocent victim: The body of a four-year old Syrian boy who was shot while fleeing the country with his family to Jordan is carried during his funeral service in Ramtha, Jordan . Syrian rebels clash with government soldiers near Aleppo yesterday. The country's largest city could become a bloody battleground on the scale of Homs . ‘This is the concern: that we will see . a massacre in Aleppo, and that’s what the regime appears to be lining . up for,’ a spokesman said. A captured policeman in Aleppo . British Foreign Secretary William Hague . warned: ‘This utterly unacceptable escalation of the conflict could lead . to a devastating loss of civilian life and a humanitarian disaster. ‘All those with influence on the . Syrian regime should bring it to bear now. No nation should stand silent . while people in Aleppo are threatened with a potential massacre.’ He said the looming battle showed why . Russia and China were wrong to veto a UN Security Council resolution . that would have imposed new sanctions backed with the threat of military . intervention. As the remaining residents of Aleppo . braced themselves for more bloodshed, General Robert Mood, the outgoing . head of the UN monitoring mission, said Assad’s days were numbered. ‘It is only a matter of time before a . regime that is using such heavy  military power and disproportional . violence against the civilian population is going to fall,’ said the . Norwegian general. Rebels have detained at least 100 . Syrian soldiers, policemen and pro-government militiamen this week in . Aleppo, according to the Syrian Observatory for Human Rights. Some of the detainees’ faces were . covered with bruises and showed signs of swelling around their eyes and . when they spoke their voices quavered. Yesterday government troops stationed . on the outskirts of the city unleashed barrages of heavy-calibre mortar . rounds on its western neighbourhoods, while Russian-built  MI-25 . helicopter gunships struck in the east. A Free Syrian Army fighter looks out from the window of the burnt Shaar district police station in the northern Syrian city of Aleppo after it was overrun by rebel fighters on Wednesday . Prepared: A truck loaded with boxes labelled gas masks sits parked outside a rebel forces base in Aleppo . Scene of destruction: The Juret al-Shayah district of the central city of Homs lies in a state of ruin . The heavy fighting follows an . audacious bomb attack that killed four of Assad’s closest lieutenants in . Damascus on July 18, a development that led some analysts to speculate . that the government’s grip was slipping. Majed al-Nour, an opposition activist, . said: ‘The rebels are present in the east and west of Aleppo, and have a . foothold in areas of the centre. ‘The regime forces control the . entrances of Aleppo and the main thoroughfares and commercial streets . and are bombarding the residential districts that fell into rebel . hands.’ He added that tens of thousands of people had fled Aleppo to . nearby northern rural regions close to Turkey. Homs in ruins. Aleppo is now set for a similarly relentless assault as it emerged that 34 people were killed in and around the city yesterday . Shelter: The Domiz camp for Syrian refugees in the northern Iraqi province of Dohuk . Tens of thousands of people have already fled Aleppo to nearby northern rural regions close to Turkey and could soon end up in refugee camps like Domiz . VIDEO: Helicopter shot . down as rebels take on tanks . VIDEO: Amateur video appears to show captured Syria loyalists in Aleppo . VIDEO: Ongoing assault could be catastrophic. Ban Ki-moon and William Hague call for end to violence .
U.S. State Department said reports of tank columns moving on Aleppo, along with air strikes, represent serious escalation of Assad's efforts to crush rebellion .
Warning comes as former head of UN monitoring mission in Syria said it is just a matter of time before Assad's government falls because its use of massive force is mobilising insurgents .
David Cameron urges Syrian regime to 'stop what it is doing' in Aleppo .
By . Mail Foreign Service . PUBLISHED: . 07:40 EST, 27 July 2012 . | . UPDATED: . 20:15 EST, 27 July 2012 . Syrian government forces yesterday launched an attack on the country’s largest city in what Britain and the U.S. warned may become a massacre. Artillery fire and an air bombardment pounded rebel-held areas of Aleppo to prepare the ground for a full onslaught. The rebels have been gradually seizing parts of Syria’s most strategically-important city, and for the past week fighting fierce  battles with President Bashar al-Assad’s forces. Scroll down for video . Syrian rebels patrol on the outskirts of Aleppo yesterday. The U.S. haswarned of a massacre in the city, after President Bashar Assad's forces launched ground and air bombardments in preparation for a major onslaught . Unsafe: Syrian civilians leave their homes in Aleppo during a lull in fighting between opposition forces and army troops . Free Syrian Army fighters battle government security forces during the siege of the Shaar district police station in Aleppo, which was later overrun by the rebel forces yesterday . But the pro-government al-Watan . newspaper warned that the ‘mother of all battles’ was about to start as . Assad’s regime tries to recapture all of the city. The battle for . Aleppo, which is home to 2.5million, is being seen as a turning point in . the 16-month uprising against Assad in a conflict where both the rebels . and the government have struggled to gain the upper hand. The US State Department said it had . received credible reports of tank columns moving on Aleppo, along with . air strikes by helicopter gunships. These represented a serious . escalation of Assad’s efforts to crush his opponents. Taken from a video uploaded to YouTube, this image purports to show members of Assad forces being held prisoner by rebels in Allepo . Innocent victim: The body of a four-year old Syrian boy who was shot while fleeing the country with his family to Jordan is carried during his funeral service in Ramtha, Jordan . Syrian rebels clash with government soldiers near Aleppo yesterday. The country's largest city could become a bloody battleground on the scale of Homs . ‘This is the concern: that we will see . a massacre in Aleppo, and that’s what the regime appears to be lining . up for,’ a spokesman said. A captured policeman in Aleppo . British Foreign Secretary William Hague . warned: ‘This utterly unacceptable escalation of the conflict could lead . to a devastating loss of civilian life and a humanitarian disaster. ‘All those with influence on the . Syrian regime should bring it to bear now. No nation should stand silent . while people in Aleppo are threatened with a potential massacre.’ He said the looming battle showed why . Russia and China were wrong to veto a UN Security Council resolution . that would have imposed new sanctions backed with the threat of military . intervention. As the remaining residents of Aleppo . braced themselves for more bloodshed, General Robert Mood, the outgoing . head of the UN monitoring mission, said Assad’s days were numbered. ‘It is only a matter of time before a . regime that is using such heavy  military power and disproportional . violence against the civilian population is going to fall,’ said the . Norwegian general. Rebels have detained at least 100 . Syrian soldiers, policemen and pro-government militiamen this week in . Aleppo, according to the Syrian Observatory for Human Rights. Some of the detainees’ faces were . covered with bruises and showed signs of swelling around their eyes and . when they spoke their voices quavered. Yesterday government troops stationed . on the outskirts of the city unleashed barrages of heavy-calibre mortar . rounds on its western neighbourhoods, while Russian-built  MI-25 . helicopter gunships struck in the east. A Free Syrian Army fighter looks out from the window of the burnt Shaar district police station in the northern Syrian city of Aleppo after it was overrun by rebel fighters on Wednesday . Prepared: A truck loaded with boxes labelled gas masks sits parked outside a rebel forces base in Aleppo . Scene of destruction: The Juret al-Shayah district of the central city of Homs lies in a state of ruin . The heavy fighting follows an . audacious bomb attack that killed four of Assad’s closest lieutenants in . Damascus on July 18, a development that led some analysts to speculate . that the government’s grip was slipping. Majed al-Nour, an opposition activist, . said: ‘The rebels are present in the east and west of Aleppo, and have a . foothold in areas of the centre. ‘The regime forces control the . entrances of Aleppo and the main thoroughfares and commercial streets . and are bombarding the residential districts that fell into rebel . hands.’ He added that tens of thousands of people had fled Aleppo to . nearby northern rural regions close to Turkey. Homs in ruins. Aleppo is now set for a similarly relentless assault as it emerged that 34 people were killed in and around the city yesterday . Shelter: The Domiz camp for Syrian refugees in the northern Iraqi province of Dohuk . Tens of thousands of people have already fled Aleppo to nearby northern rural regions close to Turkey and could soon end up in refugee camps like Domiz . VIDEO: Helicopter shot . down as rebels take on tanks . VIDEO: Amateur video appears to show captured Syria loyalists in Aleppo . VIDEO: Ongoing assault could be catastrophic. Ban Ki-moon and William Hague call for end to violence .
Department said it had . received credible reports of tank columns moving on Aleppo, along with . air strikes by helicopter gunships. These represented a serious . escalation of Assad’s efforts to crush his opponents. Taken from a those with influence on the . Syrian regime UN monitoring mission, said Assad’s days were numbered. ‘It is only a matter of time before a .