Syrian Republican Guard forces pushed into the rebel-held area of Douma in eastern Ghouta near Damascus on Friday, Syrian state TV reported, as mediated talks between the combatants collapsed.
The reported advance followed heavy air strikes on Douma town, the TV and a war monitor said.
There was no immediate comment from Jaish al-Islam, the rebel group holding out in Douma after insurgents in other parts of eastern Ghouta accepted safe passage out to opposition-held areas at the border with Turkey.
"The negotiations have ended with failure. As far as Douma is concerned, resolving it militarily is the solution," a commander in the regional military alliance that backs Assad told Reuters.
The conquest of Douma would seal President Bashar al-Assad's biggest victory over the rebellion since 2016, and underline his unassailable position in the conflict that mushroomed out of protests against his rule seven years ago.
Footage broadcast by state TV showed clouds of smoke rising from the area where the Syrian Observatory for Human Rights says tens of thousands of people are estimated to be sheltering.
The Observatory said two people had been killed and 25 wounded in a dozen air raids against Douma. It said the warplanes were likely Russian.
The Pro-opposition Orient TV said more than 20 people had been killed in the air strikes.
State media said a child was killed an 15 civilians wounded in Jaish al-Islam shelling of residential areas of Damascus.
State TV said Jaish al-Islam had shelled a crossing point out of Douma and a residential area, triggering the air strikes on a town where the Observatory said tens of thousands of people are still thought to be sheltering.
The Observatory said Russian-mediated talks between the government and Jaish al-Islam had collapsed, and Jaish al-Islam proposals for it to remain in the area as a local security force had been rejected.
State TV, in an on-screen news flash, said Jaish al-Islam was obstructing a deal and had refused to released abductees it has been holding.
The group has consistently rejected the idea of leaving Douma for areas near the Turkish border, saying this amounts to a policy of forced population transfer by Assad.
Several thousand people have left Douma in convoys to the north in recent days, including wounded rebel fighters and civilians but not active Jaish al-Islam fighters, Observatory Director Rami Abdulrahman said.
Russian-backed Syrian government forces launched their offensive against eastern Ghouta in February. It has been one of the most ferocious attacks of the war, killing more than 1,600 civilians in rebel-held areas, the Observatory says.
Rebels from other parts of eastern Ghouta have been moved to northern Syria in convoys.