Opposition says government or Russian jets pounded the town of Khan Sheikhoun in rebel-held Idlib.
At least 58 people, including nine children, were killed in an air raid that released "toxic gas" on the rebel-held Syrian town of Khan Sheikhoun on Tuesday, a monitor said.
Tuesday’s attack caused many people to choke or faint, and some had foam coming out of their mouths, the Britain-based Syrian Observatory for Human Rights said, citing medical sources who described it as a sign of a gas attack.
Damascus has repeatedly denied using such weapons and the Syrian army could not immediately be reached for comment.
The AFP news agency later reported, citing one if its journalists on the scene, that a rocket had slammed into a hospital where the victims were being treated, bringing rubble down on top of medics as they struggled to deal with victims.
The Observatory monitoring group, which tracks the war through a network of contacts on the ground, was unable to confirm the nature of the substance used.
Al Jazeera’s Alan Fisher, reporting from Beirut, said locals on ground expected the number of dead would increase.
"The national opposition in Syria is calling for the United Nations to launch an immediate inquiry for the security council to meet and to condemn behind this attack," he said.
Syrian government or Russian jets pounded the town of Khan Sheikhoun in rebel-held Idib in the morning, the monitoring group said.
On Sunday, suspected Russian fighter jets bombed a hospital in a city in the northern province of Idlib, wounding several people, a rescue group said.
At least ten people were wounded when three strikes targeted the main hospital in Maaret al-Numan, destroying the building, a White Helmets group official told Al Jazeera.
The White Helmets, also know as the Syrian Civil Defence, is are volunteer rescuers that operate in rebel-held territory.
"For the past week, Idlib has been targeted by ongoing air strikes, and after yesterday’s attack, one of its main hospitals has been mostly destroyed and can no longer function," Majid, another member of the White Helmets, said.
Over the past year, Doctors Without Borders has received reports of at least 71 attacks on at least 32 different health facilities, which it runs or supports in Syria.