There was a flare in fighting on Aleppo's southwestern outskirts Sunday, as Syrian opposition rebels put up a massive defense to protect the gains they made the previous night.

On Saturday, an alliance of rebel groups, including jihadist insurgents, overran a military complex in the southern Ramouseh district, breaking a three-week-old government siege on the city's opposition-controlled neighborhoods. The advance marked a major military setback for regime forces.

The British-based monitoring group, Syrian Observatory for Human Rights, reported that warplanes belonging to Syria and ally Russia responded with a series of intense air raids on Sunday.

Government forces last month took control of the only road leading to rebel-held areas in northern Aleppo. The rebels subsequently launched a counteroffensive from the city's south to open a corridor and break the blockade that had left 300,000 people trapped, according to the United Nations.

Civilians under siege

The latest rebel advance means that a major highway linking the government-controlled part of Aleppo to Damascus is now at risk. Rami Abdurrahman, the head of the Observatory, said there were fears some 1.2 million civilians in these areas could be left without a supply line.

"The western districts of Aleppo are now besieged. There are no safe routes for civilians in government-held districts to use to get into or out of the city," he told the Agence France-Presse news agency. He added that food prices in government-held parts of the city had already gone up.

Once an economic powerhouse, Aleppo has been divided between government forces in the west and rebels in the east since fighting for control of the city broke out in 2012. Syria's war began in 2011 with protests against the regime of Syrian President Bashar al-Assad. It has since evolved into a multisided conflict that has drawn in world players and killed more than 280,000 people.

nm/jlw (Reuters, AFP, AP, dpa)