Sunday's strikes came despite reports of a ceasefire and the potential resumption of talks between Syria's regime and Jaish al-Islam, the last rebel faction in Ghouta.
Allegations of a chlorine gas attack on Saturday were causing widespread international concern, but Syrian state media and regime ally Russia denounced the claims as "fabrications".
Assad's forces renewed their assault on Douma, the last rebel-controlled town in Eastern Ghouta, on Friday after talks over an evacuation of Jaish al-Islam fighters broke down.
The regime has used a fierce military onslaught and two negotiated withdrawals to retake control of 95 percent of Eastern Ghouta, once the main rebel stronghold close to Damascus.
It appeared last week that Douma would follow suit, with the evacuation of hundreds of rebels and their families, but there were reports of divisions among the rebels with hardliners refusing to go.
At least 80 civilians have been killed since Friday after the regime launched fresh air raids, according to the Syrian Observatory for Human Rights, a Britain-based monitor.
'Horrifying'
The Observatory also reported that dozens of people had suffered breathing problems following the attacks, but could not identify the cause.
The White Helmets, who act as first responders in rebel-held areas of Syria, alleged that regime forces had used "poisonous chlorine gas" in the attacks.
Footage posted by the group online, which was not possible to verify, showed victims including children foaming at the mouth.
There were conflicting reports on the number of dead in the alleged gas attack, with the White Helmets reporting between 40 and 70 killed.
The Observatory said that 11 of those who died at the weekend, including four children, had suffered breathing problems after the raids.
"These reports, if confirmed, are horrifying and demand an immediate response by the international community," US State Department spokeswoman Heather Nauert said in a statement.
"The Assad regime and its backers must be held accountable," she said. "Russia, with its unwavering support for the regime, ultimately bears responsibility for these brutal attacks."
The Syrian regime has been repeatedly accused of using chemical weapons, with the United Nations among those blaming government forces for a deadly sarin gas attack on the opposition-held village of Khan Sheikhun in April 2017.
That attack prompted Washington to launch military strikes against a regime military base.
Major General Yuri Yevtushenko, head of the Russian Centre for Reconciliation of the Warring Sides in Syria, denied the claims.
"We are ready, once Douma is freed from militants, to immediately send Russian specialists in radiation, chemical and biological defence to collect data that will confirm these claims are fabricated," he said in comments reported by Russian news agencies.
Resumption of talks
On Sunday morning, a civilian committee taking part in the talks between the rebels and Russia announced "a ceasefire and the resumption of talks today" hoping it will lead to a "final accord".
State news agency SANA also reported that talks would begin within several hours.
Assad is keen to recapture Ghouta to eliminate the opposition from the outskirts of Damascus and end years of rocket fire on the capital.
Since February 18, the regime's Ghouta offensive has killed more than 1,600 civilians and sliced the area into three isolated pockets, each held by different rebel factions.
The first two were evacuated under Russian-brokered deals that saw more than 46,000 rebels and civilians bussed to opposition-held Idlib province in the northwest.
Tens of thousands also fled into government-controlled territory through safe passages opened by Russia and Syrian troops.
Moscow also stepped in to negotiate a deal for Douma, the third and final pocket where Jaish al-Islam had been angling for a reconciliation agreement that would allow its members to remain as a police force.
Following a preliminary accord announced by Russia last Sunday, nearly 3,000 fighters and civilians were evacuated from Douma to northern Syria.
But as talks dragged on, Syria and Russia threatened Jaish al-Islam with a renewed military assault if the group did not agree to withdraw.
Eyewitness? Submit your stories now via social or:
Email: eyewitness@pulse.ng
