A suspected gas attack, believed to be by Syrian government jets, has killed at least 58 people including 11 children under the age of eight in the northwestern province of Idlib, a war monitor and medical workers in the rebel-held area said.
A Syrian military source strongly denied the army had used any such weapons.
The attack caused many people to choke or faint, and some had foam coming out of their mouths, the Syrian Observatory for Human Rights said, citing medical sources who described it as a sign of a gas attack.
The air strikes on the town of Khan Sheikhoun, in the south of rebel-held Idlib, also wounded more than 60 people, said the Observatory, a British-based war monitoring group.
"This morning, at 6.30 am, warplanes targeted Khan Sheikhoun with gases, believed to be sarin and chlorine," said Mounzer Khalil, head of Idlib's health authority, adding that the attack had killed more than 50 people and wounded 300.
"Most of the hospitals in Idlib province are now overflowing with wounded people," he told a news conference in Idlib.
It would mark the deadliest chemical attack in Syria since sarin gas killed hundreds of civilians in Ghouta near Damascus in August 2013. Western states said the Syrian government was responsible for that attack. Damascus blamed it on rebels.
Activists in northern Syria circulated pictures on social media showing a purported victim with foam around his mouth, and rescue workers hosing down almost naked children squirming on the floor.
The Russian Defence Ministry said on Tuesday that Russian planes had not carried out air strikes on Idlib.
