At least 18 civilians, including two children, were killed in an air strike early Tuesday that released "toxic gas" on a rebel-held town in northwest Syria, a monitor said.
The Syrian Observatory for Human Rights said those killed in the town of Khan Sheikhun, in the central province of Idlib, had died from the effects of the gas, adding that dozens more suffered respiratory problems and other symptoms.
The Britain-based monitoring group was unable to confirm the nature of the substance, and said it was unclear if the planes involved in the attack were Syrian or those of government ally Russia.
The Observatory, which relies on a network of sources inside Syria for its information, says it determines whose planes carry out raids according to type, location, flight patterns and munitions used.
It said medical sources in the town reported symptoms among the affected including fainting, vomiting and foaming at the mouth.
Photographs circulated by activists showed members of the volunteer White Helmets rescue group using hoses to wash down the injured, as well as at least two men with white foam around their mouths.
Idlib province is largely controlled by an alliance of rebels including former Al-Qaeda affiliate Fateh al-Sham Front.
It is regularly targeted in strikes by the regime, as well as Russian warplanes, and has also been hit by the US-led coalition fighting the Islamic State group, usually targeting jihadists.
Syria's government officially joined the Chemical Weapons Convention and turned over its chemical arsenal in 2013, as part of a deal to avert US military action.
But there have been repeated allegations of chemical weapons use by the government since then, with a UN-led investigation pointing the finger at the regime for at least three chlorine attacks in 2014 and 2015.
The government denies the use of chemical weapons and has in turn accused rebels of using banned weapons.