Medics in the rebel-held Eastern Ghouta area of Syria say they have been treating people with breathing problems after a suspected chlorine attack.
The reports follow government air strikes and shelling just hours after the last UN aid envoy left the enclave following a supposed five-hour truce.
Syria denied allegations it had carried out the attack, dismissing them as an act of desperation by Western powers.
Russia meanwhile has offered rebels safe passage out of the area.
The latest suspected chemical attack targeted the town of Hammoria and led to the evacuation of dozens of civilians, according to the Syrian Civil Defence, whose volunteer rescue workers are widely known as the White Helmets.
The group tweeted that “more 30 suffocation cases”, including women and children, were reported following what it described as a “regime gas attack”.
The UK-based Syrian Observatory for Human Rights (SOHR) monitoring group, which uses a network of sources on the ground, said it could not specify the cause of the illnesses.
Syrian state media suggested the attack had been staged amid “desperate attempts by some Western countries in the [UN] Security Council and the Human Rights Council to rescue terrorists who will face their inevitable end at the hands of the Syrian Arab Army”.
The UN Human Rights Council has ordered an investigation into the recent violence, and has already condemned the alleged use of chemical weapons.
Syrian President Bashar al-Assad has dismissed dire assessments of the humanitarian situation in the enclave as “ridiculous lies”.
On Tuesday, the Russian military said in a statement that its forces would provide transport and security for the rebels and their families looking to leave the besieged enclave.
The offer, which would allow rebels with personal weapons through a secure corridor, could pave the way for a deal by which the opposition would surrender its last major stronghold near Damascus to Mr Assad.
A spokesman for one of the dominant rebel groups in Eastern Ghouta, Faylaq al-Rahman, accused Russia – Syria’s key ally – of military escalation and of trying to displace the population by force.
On Monday, 94 civilians were killed amid Syrian government air strikes in Eastern Ghouta, despite a supposed five-hour truce, according to the Union of Medical Care and Relief Organizations (UOSSM), which operates medical facilities there.
A UN convoy delivering the first aid since February to the town of Douma was forced to cut short its mission and left after nearly nine hours amid shelling. The UN’s refugee agency said 10 out of more than 40 trucks had not been emptied.
The Syrian Red Crescent earlier said 46 truckloads of food parcels were delivered to 27,500 people, along with health items for more than 70,000 people in Douma.
More than 700 people have been killed in violence in Eastern Ghouta since the government intensified its offensive in recent weeks, many of them children.
Several reports suggest government forces are attempting to cut the region – home to about 393,000 people – in two.