|A civil defence member breathes through an oxygen mask, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun, Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah
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.
"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, 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.
|A man breathes through an oxygen mask as another one receives treatments, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah
The Syrian military source on Tuesday denied allegations that government forces had used chemical weapons, dismissing the accounts as rebel propaganda.
The army "has not and does not use them, not in the past and not in the future, because it does not have them in the first place", the source said.
The Russian Defence Ministry said on Tuesday that Russian planes had not carried out air strikes on Idlib.
Idlib province contains the largest populated area controlled by the anti-Assad rebels - both nationalist Free Syrian Army groups and Islamist factions including the former al Qaeda-linked Nusra Front.
U.S. air strikes since January have also hit several areas in the rural province where jihadists have a powerful presence.