The men, women and children lying undisturbed in their beds had looked so peaceful they might have been just sleeping, Abu Nidal thought, as he and other rescuers dragged their bodies into the street.

His was one of many accounts of a massive assault on the eastern suburbs of Damascus that activists say killed more than 500 people yesterday morning. They say some of the bombs were loaded with chemical agent, which would make it the worst chemical attack since Syria’s two-year conflict began.

“We would go into a house and everything was in its place, every person was in their place. They were lying where they had been. They looked like they were asleep. But they were dead,” he told Reuters by Skype from the suburb of Erbin.

When shelling hit her town of Mouadamiya, southwest of the capital, Farah al-Shami ignored the rumours on Facebook that rockets were loaded with chemical agents. She thought her district was too close to a military encampment to be affected.

“And at the same time the UN was here. It seemed impossible. But then I started to feel dizzy. I was choking and my eyes were burning,” the 23-year-old said.

“I rushed to the field clinic nearby. Luckily no one in my family was hurt, but I saw entire families on the floor.”

Reuters was unable to verify witness accounts. Doctors interviewed in the area describe symptoms they believe point to sarin gas, one of the agents Western powers accuse Damascus of having in an undeclared chemical weapons stockpile.

The government denies having such weapons and says that if it did posses them, it would not use them against other Syrians. It repeated those statements again yesterday, calling the reports of the gas attack “fabrications”.

Videos from the suburbs of the rebel stronghold outside the capital, known as Ghouta, show similar scenes of small children convulsing on the floor, foaming at the nose and mouth. Some show rows of men, women and children – pale but with no signs of injury – laid out for burial.