Russian air strikes killed 37 civilians in the Arbin area of the shrinking rebel enclave of Eastern Ghouta near Damascus overnight, the Syrian Observatory for Human Rights said on Friday.
"Russian air strikes and incendiary weapons killed the civilians in a basement from burning or suffocation" late on Thursday before a ceasefire came into effect in the area, Observatory head Rami Abdel Rahman said.
Russia has denied being directly involved in air strikes on Eastern Ghouta.
The Britain-based Observatory says it relies on flight patterns, aircraft involved and ammunition used to determine who carries them out.
The White Helmets, a civil defence organisation operating in rebel-held areas, said most of the dead were women and children.
In an image they shared on social media, rescue workers wrapped a blackened body in a blanket.
More than 1,600 civilians have died in Eastern Ghouta since the regime launched a blistering assault on the last rebel bastion near Damascus on February 18, the Observatory says.
The offensive has retaken most of the enclave and divided what remains into three shrinking pockets, each controlled by a different rebel group.
Late on Thursday, the Faylaq al-Rahman group which controls the southern pocket that includes Arbin, said a ceasefire had been agreed from midnight (2200 GMT) to allow negotiations with Russia for an evacuation deal.
Under a similar agreement reached by the Ahrar al-Sham group, hundreds of rebel fighters and their family members were evacuated from the town of Harasta earlier on Thursday.
The ceasefire announcement came after air strikes killed 38 people in Arbin and Zamalka, another town controlled by Faylaq al-Rahman, earlier on Thursday, the Observatory said.