The Syrian Observatory for Human Rights says airstrikes and artillery fire have killed dozens of people in a rebel enclave in eastern Ghouta.
Ro Aram reports.
The war monitoring group said on Monday that at least 77 people were killed in the space of 24 hours.
325 people were also said to have been wounded.
The British-based Observatory said at least 20 children were among the civilians killed in the assault.
Syrian state media said Sunday's attack came after rebel factions in Ghouta fired mortars at districts in Damascus, killing a child and wounding eight others.
The government airstrikes and artillery fire also came as pro-regime forces prepare to launch a major ground assault on Eastern Ghouta.
They are reportedly trying to help the Kurdish YPG militia try to repel a month-old Turkish military assault in Afrin, dubbed Operation Olive Branch.
But, the YPG has denied there's a deal with Damascus.
Turkey's foreign minister said the Syrian army's efforts to defend the YPG would come to nothing, adding that Turkish soldiers will not be stopped.
There was no comment from the Syrian military and the Damascus government says it only targets militants.
Eastern Ghouta is home to nearly 400-thousand people, according to the UN, and is the largest rebel stronghold left near the capital.
Hundreds of thousands of people have died in fighting during Syria's seven-year civil war, and millions have fled the country.
Ro Aram, Arirang News.