Censor.NET reports citing Reuters referring to a monitor, medics and rescue workers in the rebel-held area.
A Syrian military source strongly denied the army had used any such weapons.
The Syrian Observatory for Human Rights said the attack, believed to have been carried out by Syrian army jets, caused many people to choke, and some had foam coming out of their mouths. All the children were under the age of eight.
"This morning, at 6:30 a.m., warplanes targeted Khan Sheikhoun with gases, believed to be sarin and chlorine," said Mounzer Khalil, head of Idlib's health authority. The attack had killed more than 50 people and wounded 300, he said.
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