The alleged attack in the town of Douma occurred late Saturday amid a resumed offensive by Syrian government forces after the collapse of a truce.
The Syrian government and its ally, Russia, however, denied any involvement in the alleged gas attack.
Maj Gen Yuri Yevtushenko was quoted by Russian news agencies on Sunday as saying Russia was prepared to “promptly send Russian specialists in radiation, chemical and biological protection to Douma after its liberation from fighters to gather data that will confirm the fabricated nature of these statements”.
Yevtushenko said “a number of Western countries” are trying to prevent the resumption of an operation to remove Army of Islam fighters from Douma and “to this end they are using the West’s pet theme of the use of chemical weapons by Syrian forces”.
Russia is a key ally of President Bashar Assad, whose forces have been accused of using chemical weapons in past attacks that killed hundreds of people.
Opposition-linked first responders, known as the White Helmets, reported the attack, saying entire families were found suffocated in their homes and shelters. It reported a death toll from suffocation of more than 40, saying the victims showed signs of gas poisoning including pupil dilation and foaming at the mouth. In a statement, however, it reported a smell resembling chlorine, which would not explain the described symptoms, usually associated with sarin gas.
It said around 500 people were treated for suffocation and other symptoms, adding that most medical facilities and ambulances were put out of service because of the shelling.
The Syrian American Medical Society, a relief organization, said 41 people were killed and hundreds wounded.
The Syrian Observatory for Human Rights said at least 80 people were killed in Douma — the last rebel stronghold in eastern Ghouta — on Saturday, including around 40 who died from suffocation. But it said the suffocations were the result of shelters collapsing on people inside.
Videos posted online by the White Helmets purportedly showed victims, including toddlers in diapers, breathing through oxygen masks at makeshift hospitals.
The Syrian government, in a statement posted on the state-run news agency SANA, strongly denied the allegations. It said the claims were “fabrications” by the Army of Islam rebel group, calling it a “failed attempt” to impede government advances.
“The army, which is advancing rapidly and with determination, does not need to use any kind of chemical agents,” the statement said.
Syrian government forces resumed their offensive on rebel-held Douma on Friday afternoon after a 10-day truce collapsed over disagreement regarding the evacuation of Army of Islam fighters. Violence resumed days after hundreds of opposition fighters and their relatives left Douma toward rebel-held areas in northern Syria.
