Nineteen children are among the dead after a round of airstrikes by the Syrian government on a rebel enclave near Damascus.
The town of Douma in Eastern Ghouta was under siege on Wednesday, with the Syrian Observatory for Human Rights reporting that 34 people had been killed and about 200 more injured in the bombardment.
It came less than 24 hours after the region – controlled by rebel factions, including Islamists – suffered some of its worst bloodshed in years, with 80 civilians left dead by regime warplanes.
Rami Abdel Rahman, director of the UK-based observatory, said Tuesday had seen “the highest civilian toll in Syria in nearly nine months, and one of the bloodiest days in Eastern Ghouta for several years”.
Beit Sewa and Hammuriyeh, two small towns in the district, were the hardest hit, with some of the munitions thought to contain chlorine.
The global chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons, expressed “grave concern” over the “credible allegations”.
Syria’s government has repeatedly denied using chemical weapons, but the US State Department says it has recorded at least six suspected chemical attacks in the country in the past month.
In apparent retaliation for this week’s strikes, rebel rockets have been fired on a government-controlled suburb of Damascus, killing one child and two other people.
State news agency SANA reported that the shells were launched towards the capital from Eastern Ghouta, which is home to 400,000 people and had been billed as a de-escalation zone.
The UN’s humanitarian coordinator for Syria, Panos Moumtzis, said on Wednesday that he was concerned over the continued violence across the war-ravaged nation.
He said: “There is a misperception that de-escalation areas have resulted in peace and stability.
“Eastern Ghouta is a de-escalation area. If anything, there has been a serious escalation. The conflict in Syria is far from over. It’s the first time – between Eastern Ghouta, Idlib, Afrin – we have multiple fronts with people in extreme danger without a view to a solution.”
More than 340,000 people have been killed and millions displaced since the conflict in Syria erupted in March 2011.