At least 58 people, including nine children, were killed in an air raid that released "toxic gas" on the opposition-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.

The monitoring group, which tracks the war through a network of contacts on the ground, was unable to confirm the nature of the substance.

Al Jazeera's Alan Fisher, reporting from Beirut, said the locals on ground expect the number of dead might increase.

"The national opposition in Syria is calling for UN to launch an immediate inquiry for the security council to meet and to condemn behind this attack," he said.

Syrian regime or Russian jets pounded the town of Khan Sheikhoun in opposition-held Idib in the morning, the monitoring group said.

The Syrian regime army could not immediately be reached for comment.

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 Syrian Civil Defense official told Al Jazeera.

The Syrian Civil Defense, a rescue group also known as the White Helmets, operates in opposition 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 Syrian Civil Defence, 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.

PHOTO CAPTION

A civil defense member breathes through an oxygen mask, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in opposition-held Idlib, Syria April 4, 2017. REUTERS

Al-Jazeera