On another front in Syria's seven-year civil war, Turkish airstrikes killed 36 pro-regime fighters in a Kurdish enclave near Turkey's border.
Syria's war has killed more than 340,000 people and spiraled into a complex conflict involving world powers since it started in 2011 with the brutal repression of anti-government protests.
Russia-backed regime forces have since February 18 killed more than 640 civilians - including more than 150 children - in a bombardment of Eastern Ghouta, one of the armed opposition's last strongholds in the country.
Following a deadly wave of airstrikes and shelling, fighting on the ground has intensified in recent days, and the regime now "controls 10 percent of the besieged Eastern Ghouta region," the Syrian Observatory for Human Rights said late Saturday.
Regime forces have advanced steadily since Thursday, the Britain-based monitor said earlier, recapturing four areas in the east and southeast of the enclave as well as two air bases.
"Regime forces and their allies have intensified their attacks on rebel positions in the past 48 hours," Observatory head Rami Abdel Rahman said.
The progress has been rapid through mainly agricultural land, he added.
"The terrorists will soon taste defeat in Ghouta," said Syria's Deputy Foreign Minister Ayman Soussan.
Eastern Ghouta's 400,000 residents have lived under siege since 2013, facing severe food and medicine shortages even before the latest offensive.
The fighting on the ground comes after Russia on Tuesday started a daily five-hour "humanitarian pause" in the enclave: a move that falls short of the 30-day cease-fire demanded by the UN Security Council.
Trucks loaded with desperately needed aid remain unable to enter the enclave. Thousands of civilians are surviving on meager stocks, and medical staff are struggling to treat casualties with inadequate medical supplies.