The United States and Russia on Sunday tabled a proposal to rebels in Aleppo that would offer safe passage from the city for fighters, their families and other civilians, three opposition officials with Aleppo rebel groups told Reuters.

The rebel groups in Aleppo have yet to respond to the proposal, the officials said. The proposal promised rebel fighters a “secure” and “honourable” withdrawal from the city, they said.

If rebels accept the proposal, it would restore Syrian President Bashar al-Assad’s full control over rebel-held areas of eastern Aleppo, his biggest victory yet in the civil war that has shattered his country.

The Russian-backed Syrian military and its allies have captured swathes of rebel-held eastern Aleppo in a ferocious military campaign, squeezing rebel fighters and tens of thousands of civilians into an ever shrinking enclave.