Eyewitnesses said the attack was carried out by jets operated by the Russian and Syrian governments, but both Moscow and Damascus denied responsibility.
Photos and video emerging from Khan Sheikhoun, which lies south of the city of Idlib, the provincial capital, show limp bodies of children and adults. Some are seen struggling to breathe; others appear foaming at the mouth.
The activist-run Assi Press published video of paramedics carrying victims from the scene by a pickup truck. The victims were stripped down to their underwear. Many appeared unresponsive.