A Syrian rebel group accused government forces on Saturday of dropping a barrel bomb containing poisonous chemicals on civilians in eastern Ghouta, and a medical relief organization said 35 people had been killed in chemical attacks on the area.

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.

On Saturday evening, the U.S. State Department said it was monitoring the situation and that Russia should be blamed if chemicals were used.

The Syrian Observatory for Human Rights said 11 people had died in Douma as a result of suffocation caused by the smoke from conventional weapons being dropped by the government. It said a total of 70 people suffered breathing difficulties.

Rami Abdulrahman, the Observatory director, said he could not confirm if chemical weapons had been used.

Medical relief organisation Syrian American Medical Society (SAMS) said a chlorine bomb hit Douma hospital, killing six people, and a second attack with "mixed agents" including nerve agents had hit a nearby building.

"The regime's history of using chemical weapons against its own people in not in dispute," said a State Department official in a statement. "Russia ultimately bears responsibility for the brutal targeting of countless Syrians with chemical weapons."

Basel Termanini, the U.S.-based vice president of SAMS, told Reuters the total death toll in the chemical attacks was 35. "We are contacting the U.N. and the U.S. government and the European governments," he said by telephone.

Reuters could not independently verify the reports.

Syrian state news agency SANA said the rebel group in Douma, Jaish al-Islam, was making "chemical attack fabrications in an exposed and failed attempt to obstruct advances by the Syrian Arab army," citing an official source.