A deadly Al Qaeda-linked terror attack on a Syrian regime intelligence headquarters yesterday was so massive it measured 2.3 on the Richter scale and registered as an earthquake. At least 20 regime security force members and 14 rebels were killed in the west of Aleppo - the largest city in Syria and the frontline in battles between the Assad regime, the Islamic State, Western-backed rebel groups and the Al Qaeda-affiliated Nusra Front. The latter group are believed to have carried out yesterday's attack targeting air force intelligence offices, in which they used a tunnel to detonate bombs, creating an explosion so huge that the European Mediterranean Seismological Centre mistook it for an earthquake. Scroll down for video . Attack: The citadel (background) in the northern Syrian city of Aleppo was where 34 people were killed yesterday when Al Qaeda-backed rebels set off a powerful tunnel explosion targeting an intelligence HQ . Huge: Yesterday's terror attack created an explosion so massive that the European Mediterranean Seismological Centre mistook it for an 2.3 magnitude earthquake . Watching the destruction: Members of Syrian opposition forces are seen hiding behind rubble after they blow up the tunnel beneath the Syrian Air Intelligence Directorate's Aleppo branch . Yesterday's attack came just days after the opposition rejected a UN ceasefire plan for the city and as President Bashar al-Assad insisted he continues to enjoy the support of the Syrian people despite nearly four years of war and international pressure on his regime. 'The goal was to storm the building and to control it, but they failed,' Rami Abdel Rahman, the director of UK-based Syrian Observatory for Human Rights monitoring group . Six civilians were also killed in separate rebel shelling of regime-controlled areas, he said. The attack began with a huge blast from explosives in a tunnel near the intelligence building, the Observatory and a Syrian military source said. 'Gunmen blew up a tunnel that they dug (into the regime-controlled sector) and then attacked the area surrounding the air force intelligence headquarters,' the military source said. Journalists in eastern Aleppo said the blast was heard across the city. It later emerged that the explosion was so huge that it measured 2.3 on the Richter scale and was registered as an earthquake by the European Mediterranean Seismological Centre. Sniper: A member of the Syrian opposition force is seen after blowing up the tunnel under the intelligence HQ . Smoke rises after Syrian opposition forces blow up the Syrian Air Intelligence Directorate's Aleppo branch . Carnage: The Syrian Air Intelligence Directorate's Aleppo branch is seen going up in smoke after the attack . Rebels from several factions then launched on assault on the building, part of which had collapsed from the explosion, said the Observatory, which relies on a network of sources inside Syria. The rebels faced heavy resistance from government troops supported by fighters from Lebanese Shiite militant group Hezbollah, a key backer of Assad, it said. The assault was repelled with the help of regime air strikes on rebel positions and the clashes eventually subsided. 'Dozens of (rebel) gunmen were killed in artillery and air strikes. The situation is quiet now in the area. There are minor sporadic clashes,' the Syrian military source said. Al-Qaeda's affiliate in Syria, Nusra Front, said on Twitter that its forces, along with other rebel factions, had 'stormed the air force intelligence offices and surrounding buildings'. The attack was the worst reported violence in Aleppo since the rebels  rejected a UN plan to freeze fighting in the northern city on Sunday. UN envoy Staffan de Mistura has made the plan for a temporary ceasefire in Aleppo the centrepiece of his efforts to bring any kind of halt to the conflict in Syria, where more than 220,000 people have been killed since it erupted in March 2011. Militants: Yesterday's attack targeting air force intelligence offices is understood to have been carried out by members of the Al Qaeda-affiliated terror group Nusra Front (pictured) On the run: Members of the rebel First Army try to hide from Assad regime forces' snipers in Aleppo . Warzone: Once Syria's commercial hub, Aleppo has been devastated by fighting that began in mid-2012, and the city is now split between loyalist forces and rebels . De Mistura in October unveiled the proposal to suspend fighting in Aleppo to allow humanitarian aid deliveries and make a first step towards a broader political deal. He held talks in Damascus on Saturday to try to finalise a deal, with a delegation member saying he hoped to set in motion as soon as possible a plan to halt fighting in Aleppo for six weeks. But rebel representatives refused to consider the proposal unless it forms the basis for a 'comprehensive solution' to the conflict through the departure of Assad. De Mistura has angered the opposition by describing Assad as 'part of the solution' to the Syrian conflict. Some Western powers have also cast doubt on the plan, with the French ambassador to the United Nations, Francois Delattre, saying Tuesday that 'France remains sceptical about the regime's willingness' to follow through on it. Blast: Civilians and civil defense members run past flames and smoke rising from a site hit by what activists said was a barrel bomb dropped by forces loyal to Syria's President Bashar al-Assad in Aleppo this morning . Flames: A civil defence member  puts out a vehicle fire caused by a Syrian regime barrel bomb today . Crunch talks: Syrian Deputy Foreign Minister Faisal Moqdad (left) meets with United Nations special envoy for Syria Staffan de Mistura (right) in Damascus last month to discuss the ongoing civil war in the country . Assad yesterday told Portuguese broadcaster RTP that the image of him presented in the West was deeply skewed. 'The people are against him, the regional countries are against him and the West is against him, and (still) he succeeded,' Assad said, speaking in English. 'You're either lying to us or you're talking about Superman because if you don't have Superman, (if) he's a regular president, it means he could withstand four years only because he has the public support.' Wednesday's attack featured a favoured tactic of Syria's rebels -- especially in Aleppo -- of digging tunnels near government buildings and setting off explosives. Fighting in Aleppo erupted in mid-2012, and control of the city -- once Syria's commercial hub -- has since been divided between rebels on the eastern side and the regime in the west. The air force intelligence headquarters in Aleppo is a key strategic site for regime forces and the surrounding area has come under repeated attack from rebel forces.
Al Qaeda-backed Nusra Front group carried out the attack west of Aleppo . The detonated explosives buried in a tunnel beneath an intelligence HQ . 20 Syrian regime security soldiers were killed along with 14 of the terrorists . Blast was so huge that many experts mistook it for a small earthquake . European Mediterranean Seismological Centre said the blast measured a magnitude of 2.3 on the Richter scale .