Amateur videos posted online showed medics trying to revive people laid out on floors and hospital gurneys with hand-pump respirators, along with images of rooms full of the lifeless bodies. The source of the images could not immediately be verified, and the government of President Bashar al-Assad denied the allegations of a chemical weapons attack.

It was unclear how many people had been killed, with estimated tolls from activists ranging from the dozens to the hundreds. The Syrian Observatory for Human Rights, which follows the conflict from Britain through a network of contacts inside Syria, said the attacks took place in the suburbs of Zamalka, Ein Terma and Erbeen, all of which are east of Damascus and have a strong rebel presence…

Unlike the videos often uploaded after government attacks, the images on Wednesday showed very little blood; few of the patients appeared to have conventional injuries. Most of the victims in the videos appeared to have suffocated.

In one video, medical workers were shown moving through what appeared to be a room full of apparently lifeless young men in their underclothing. In another, about 80 bodies, including about a dozen children and many women, were laid out on a tile floor. Other videos showed children, one of them motionless and staring, the other seeming to twitch uncontrollably.