The Syrian Observatory for Human Rights said dozens of air strikes hit various parts of Douma, including some suspected to have been carried out by Russian warplanes.
On Friday, opposition activists said 40 people were killed in Douma while state media said four were killed in government-held Damascus.
The violence came after almost two weeks of calm in the last rebel-held town in the area known as eastern Ghouta after the Russians have agreed with the Army of Islam to evacuate the area toward rebel-held regions north of the country.
Backed by Russia, Syrian troops have recaptured 95 percent of Ghouta since February 18 through a combination of a deadly air and ground assault and evacuation deals.
The members of the Jaish al-Islam group have carried out attacks on the positions of the government forces and resumed rocket and mortar shelling on Damascus...
But as talks dragged on, Syria and its Russian ally threatened Jaish al-Islam with a renewed military assault if they did not agree to withdraw.
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Syrian state TV is reporting that government forces are advancing in farms outside the rebel-held town of Douma near the capital Damascus, getting close to the last rebel-held area in eastern Ghouta.
Syrian state media quickly denied that troops had deployed chemical weapons on Douma, the last opposition-held town in the battered Eastern Ghouta enclave.
Douma is the last town held by rebels in the eastern Ghouta suburbs of Damascus.
The Army of Islam has never publicly confirmed the agreement, and is said to be divided on whether to leave Douma, with hard-liners wanting to stay and fight. "People still need me", he said.
Moscow announced a deal with Jaish al-Islam last Sunday, ushering in three consecutive days of evacuations from Douma that saw almost 3,000 fighters and civilians bussed to northern Syria.
It remains unclear exactly why the talks fell apart this week. "There were 20 strikes in 15 minutes", Mohammed, a medic, told AFP from Douma on Friday.
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"The talks were going well", he said, but power struggles between the regimes allies had caused them to break down.
The latest fatalities bring to 48 the number of civilians who have been killed in the town since Friday when the Syrian military started a major campaign on the town shattering 10 days of calm there, the Britain-based watchdog said.
It was not clear if Russian Federation, the regime's main backer, or the Syrian Air Force had conducted the latest air strikes.
On Saturday, fresh artillery fire hit those fields, said the Observatory.
"The regime is trying to tighten the noose around Douma from the west, east, and south", said Observatory chief Rami Abdel Rahman.
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