The battle for Aleppo has taken some unexpected twists in the past 48 hours, as the Syrian military — backed by Lebanese, Iranian and Russian forces — battle rebels for control of Syria's biggest prewar city.

Rebels cut the main route into government-held Aleppo on Sunday, just days after the rebels broke the government's siege of their own stronghold in the east of the city.

The rebels broke the siege on Saturday after Syrian government forces last month cut the Castello Road — the only remaining route into rebel-held held eastern Aleppo, which is home to an estimated 250,000 civilians.

As the rebels advanced south into the Ramousah area to break the government's siege, they cut the main supply route into government-controlled western Aleppo, forcing fuel, food and aid trucks keeping their enemy supplied to take an alternate route.

"It looks what we have now is a sort of double siege – two interlocking, yin-and-yang zones of control each served by a single semi-passable supply route," said Sam Heller, a Beirut-based analyst focused on Syria. "I don't think either area – opposition or regime – is entirely cut off at this point. But regime-held western Aleppo, for example, now has to be supplied via contested 'war roads' — improvised roads on which it might not be possible to sustain the swollen population of these western neighborhoods."

Heller said the result could be ugly for both sides, with a protracted battle on multiple fronts in populated areas.

Photos posted by rebels showed an opposition aid convoy that they said carried food and medical into rebel-held section of the city, the first since the government began its siege.

The Syrian Observatory for Human Rights, a London-based watchdog group, said Syrian government warplanes had bombed rebels in Ramousah after they captured the area from government forces. The Syrian military said it had also shelled the rebels in Ramousah, while the Observatory said rebels had bombarded areas of western Aleppo overnight. Syria's state news agency said nine civilians, including six children, were killed by two rebel mortar shells in the government-held area.

The Observatory says more than 130 people have been killed since the rebel offensive began, with most of the casualties in Western Aleppo.

US Permanent Representative to the United NationsSamantha Power said at a special UN meeting on the Aleppo siege that the fighting may impede the transfer of humanitarian aid to "tens or even hundreds of thousands more people in Western Aleppo," and, if the fighting continues, that "both sides of Aleppo could be cut off from the basic assistance they need."

"The fighting of the past few days confirms what we have known for a very long time – despite the overwhelming force of the Assad regime, Russia, Iran, and Hezbollah on one side – neither side will be able to win a swift or decisive victory in the battle for Aleppo," Power said. "Yet the longer the fighting drags on, the more civilians will be caught in the middle, the more that they will pay the highest price."

The rebel group that has led the offensive is Jabhat Fateh al-Sham, formerly known as the al-Qaeda affiliated Nusra Front.

Last month, Jabhat Fateh al-Sham declared it was severing its ties to al-Qaeda in order to build closer ties to other Syrian and rebel and jihadist groups.

"Unifying our efforts and ranks is imperative to meet the goals of the Syrian revolution," Jabhat Fateh al-Sham announced, saying it would lead to "a complete merger between all sincere groups." Its name means "Front for the conquest of the Levant."

Also on Monday, Doctors without Borders (MSF) said warplanes had destroyed a hospital that the organization supports in Idlib on Saturday, killing 13 people.

The United Nations has pushed for the warring parties to adhere to a ceasefire agreement hammered out earlier this year. UN Special Syria Envoy Stefan Staffan de Mistura plans to hold intra-Syrian talks toward the end of August, in an effort to bring an end to the five-year-old conflict.

Previous peace talks have been unsuccessful.