Beirut - Rebels breached the Syrian government siege on opposition neighbourhoods in the city of Aleppo, opening a corridor in the south and marking a major military breakthrough. The push prompted an intensive airstrike campaign on Sunday as insurgent groups put up massive defence to protect the new corridor and gain new ground.The battle for Aleppo, Syria's largest city and former commercial heart, is pivotal for the Syrian civil war. It is not clear whether the rebels would be able to keep their new gains, but the breach causes a dent in the Syrian government's new confidence and territorial expansion, bolstered by Russian air support.Global terror networkIn a major offensive on Saturday, an alliance of over two dozen rebel groups pushed government forces and allied fighters out of parts of the southern Ramouseh district, including from a number of military colleges, a bakery, a post office, a parking lot and a section of the highway.The rebels hailed the breach as a major collaborative achievement a week after launching their counteroffensive.Fighters besieged in the city co-ordinated with the Army of Conquest alliance, which includes the Levant Conquest Front, the rebranded al-Qaeda branch in Syria, based in neighbouring Idlib province. The Levant Conquest Front had announced it was breaking ties with the global terror network a week earlier, citing as a motive the desire to improve co-ordination with local rebel groups.The media arm of Lebanon's Hezbollah, the Shi'ite militant group fighting alongside the Syrian government, conceded the rebels' advance, adding that airstrikes levelled one of the military colleges after forces withdrew.Syrian State news agency SANA, denied the siege had been broken and said the government had declared that operations were still ongoing in the area. It said warplanes targeted rebel vehicles and locations in the area on Sunday.The Levant Conquest Front posted pictures of loot from one of the military academies, the artillery school, including armoured vehicles and ammunition.Safe corridorsThe UN says there are about 18 besieged and hard-to-reach areas, almost all encircled by government forces. Last month, the Syrian government seized the Castillo road, the only route into rebel-held areas in Aleppo. The UN said 300 000 people were trapped inside, making eastern Aleppo one of the largest besieged areas in Syria. The government and major ally Russia had offered safe corridors for residents to leave rebel-held areas, an offer met with skepticism from the locals who viewed it as an attempt to depopulate the area.The rebel advance in the Ramouseh district now threatens a major highway linking the government-controlled part of Aleppo to the outside world, leaving an estimated population of 1.2 million at risk of losing a supply line. The Castillo road remains under government control but activists say it regularly comes under fire.Rami Abdurrahman, head of the Britain-based Syrian Observatory for Human Rights, said food prices in government-held parts of Aleppo have already gone up. He said some 700 fighters from the government and the insurgent side were killed in the week of fighting. State news agency SANA said one girl was killed in rebel shelling of government areas near the frontline.