At least 27 people, including five children, have been killed following heavy air strikes on the Syrian rebel-held town of Douma, the Syrian Observatory for Human Rights has said.
The monitor added that the town, which is the last held by rebels in Eastern Ghouta, had been hit with at least 42 air strikes since shelling began earlier today.
It comes as Syrian Republican Guard forces pushed into the town near Damascus as mediated talks between the rebels and government forces collapsed, according to Syrian state media.
There has been no immediate comment from 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 spokesman for government forces said.
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 monitor said the warplanes were likely Russian.
Syrian state media said a child was killed and 15 civilians wounded in rebel shelling of residential areas of Damascus.
State television said the rebels 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 the rebels had collapsed, and rebel proposals to remain in the area as a local security force had been rejected.
State TV, in an on-screen news flash, said the group, 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 it amounts to a policy of forced population transfer by President Assad.
Several thousand people have left Douma in convoys to the north in recent days, including wounded rebel fighters and civilians but not active rebel fighters, the monitor 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, according to the Observatory.
Rebels from other parts of Eastern Ghouta have been moved to northern Syria in convoys.
