The White House says it is "confident" Bashar al-Assad's government was behind an apparent chemical attack that killed at least 58 people in north-west Syria.
The Syrian Observatory for Human Rights reported that strikes on Khan Sheikhoun by Syrian government or Russian jets had caused many people to choke.
Later, aircraft fired rockets at local clinics treating survivors, medics and activists said.
Syria's army denied the government had used any such weapons.