The Syrian army is poised to slice rebel-held eastern Ghouta in two as forces advancing from the east link up with troops at the enclave's western edge, a pro-Damascus military commander says.
That put the zone effectively under Syrian government control as the remaining strip of territory was within weapons range.
The government, backed in the war by Russia and Iran, is seeking to crush the last major rebel enclave near Damascus in a ferocious campaign that the Syrian Observatory for Human Rights says has killed 893 civilians in the last 18 days, including 91 on Wednesday.
The pro-Damascus commander, who spoke on condition of anonymity, confirmed a report by the Observatory that the enclave had effectively been sliced in two.
But Wael Alwan, the Istanbul-based spokesman for Failaq al-Rahman, one of the main rebel groups in eastern Ghouta, denied that the territory had been cut in half. "No" he said in a text message when asked if the report was correct.
An aid convoy that intended to go to Ghouta later on Thursday was postponed, the International Committee of the Red Cross (ICRC) said.
The United Nations says 400,000 people are trapped in the towns and villages of eastern Ghouta. They have been under government siege for years and were already running out of food and medicine before the assault. Many civilians have fled from the frontlines into Douma, a town in the enclave.
Russia, President Bashar al-Assad's most powerful ally, has offered rebels safe passage out with their families and personal weapons. The proposal echoes previous agreements under which insurgents, in the face of military defeat, were permitted to withdraw to opposition-held areas along the Turkish border.
Russia's defence ministry said on Wednesday some rebels wanted to accept the proposal to evacuate. So far rebels have dismissed it in public and vowed to fight on.