The Syrian Observatory for Human Rights said regime warplanes on Sunday had conducted at least 10 air strikes on Douma, most of them hitting a popular marketplace.
"The toll documented by the Observatory has risen to 96 people, including at least two women and four children," the Britain-based monitoring group said early Monday.
It said another 240 people were wounded in the strikes, some of whom were in critical condition.
The Local Coordination Committees, a grassroots network of anti-regime activists, said overnight that at least 105 people had been killed in Douma, which lies in the opposition bastion of Eastern Ghouta.
The area is the regular target of government air strikes and has been under siege for nearly two years, with regime forces tightening the blockade since the start of 2015.
But Sunday's raids were one of the bloodiest regime attacks in the country's four-year war. An AFP photographer in Douma described the attack as the worst he had covered in the town.
In one makeshift clinic, whole sections of floor were covered with rows of the dead, as volunteers worked to wrap each victim in a white shroud.
The deaths came as the UN's new humanitarian chief, Stephen O'Brien, visited Syria for the first time since his appointment in May.
O'Brien met Foreign Minister Walid Muallem and expressed a willingness to work with the government to alleviate humanitarian suffering, state media said.
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