The opposition-held sector of Syria’s divided city of Aleppo has been cut off from the outside world in recent days by an escalation of air and artillery strikes on the only road in, putting hundreds of thousands of people under effective siege.

A government campaign to fully capture Aleppo would most likely bury what little hope remains of reviving a diplomatic effort to end the five-year-old civil war, after talks and a ceasefire sponsored by the United States and Russia fell apart earlier this year.

Russia said last week its air force would provide “the most active” support to prevent Aleppo and the surrounding area falling into the hands of what it called terrorists, a word Moscow and Damascus use to describe an array of Assad’s enemies.

Rebels have seen Russian statements that the al Qaeda-linked Nusra Front has mounted attacks in Aleppo as a sign Moscow is looking for new pretexts to attack: while Nusra has been active south of Aleppo, the more moderate rebels say it has no presence in the city itself. The Nusra Front, like Islamic State, was excluded from the ceasefire and peace talks that were sponsored by Washington and Moscow.

The nationalist rebels fighting Assad in Aleppo are confident of their ability to repel ground attacks in which Shi’ite militias from Iran, Afghanistan, and Lebanon’s Hezbollah have played a major role supporting the government.

A number of these rebel groups receive foreign military support from states opposed to Assad, funneled via Turkey. The rebels say they saw off three attacks by pro-government forces in less than a month on Handarat, also near the Castello road.

“The regime is trying to advance on the ground. It takes a point or two, then loses them,” said Abu Yassine, head of the Levant Front rebel group, speaking to Reuters.