More than 70 people have been killed after a suspected chemical attack on the last remaining rebel stronghold in the Eastern Ghouta area - with fears growing the toll could surpass 100, aid organisations say.
Syrian state media denied government forces had launched any chemical attack as soon as the reports began circulating and said rebels in the eastern Ghouta town of Douma were in a state of collapse and spreading false news.
The US State Department said on Saturday reports of mass casualties from an alleged chemical weapons attack in Douma were "horrifying" and would, if confirmed, "demand an immediate response by the international community".
The charity Union of Medical Care and Relief Organisations (USOSSM) told German news agency dpa that "significant numbers of children" were among those killed in the Douma attack.
In a separate statement, the charity said the death toll was expected to rise to well over 100, as rescuers had experienced "extreme difficulty reaching victims due to the continued bombardment on Douma."
"Many of the victims were women and children and experienced symptoms consistent with inhaling toxic gas," USOSSM said in a statement.
"Some of the symptoms victims experienced include: cyanosis; foaming of the mouth; cornea irritation; and the strong odour of a chlorine-like substance."
Spokesman Ari D'Souza said a smell of chlorine had been reported but that rescuers believed that sarin gas had been used, as it sinks.
In an earlier report, the volunteer rescue group White Helmets wrote on Twitter that a helicopter had dropped a barrel bomb filled with chemicals on Douma, killing at least 40 people and injuring hundreds.
The tweet was accompanied by images of apparent victims of the alleged attack, including children, with foam around their mouths.
Meanwhile Russia's Foreign Ministry said reports of a gas attack were bogus and any military action taken based on such "invented and fabricated excuses" could lead to severe consequences.
"The spread of bogus stories about the use of chlorine and other poisonous substances by [Syrian] government forces continues. Yet another such fabricated piece of information about an alleged chemical attack in Douma appeared yesterday," the ministry said in a statement.
"We have warned several times recently against such dangerous provocations. The aim of such deceitful speculation, lacking any kind of grounding, is to shield terrorists... and to attempt to justify possible external uses of force."
The offensive in Ghouta has been one of the deadliest of the seven-year-long war, killing more than 1,600 civilians according to the Syrian Observatory for Human Rights.
The Observatory said it could not confirm if chemical weapons had been used in the attack on Saturday.