A suspected chlorine-gas attack by Syrian government helicopters has killed a child and injured about 40 people in Saraqeb, activists say, in the second attack on the northwestern town in days.

Friday night's alleged attack in Saraqeb, in Idlib province, which is controlled by a coalition of opposition groups, follows reports of a similar helicopter barrage using barrel bombs on Wednesday.

It also comes a day after an international chemical-weapons watchdog said it was ready to investigate a series of recent chemical attacks.

Videos shared by the Syrian Civil Defence activist group showed medics and residents rushing children to a local hospital as they coughed, some gasping for air.

A video from Nareb, another town in the province where fighters supported by Turkey, Qatar and Saudi Arabia have made gains in recent days against troops loyal to President Bashar al-Assad, showed a medic receiving oxygen himself after rescuing people from another attack.

The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists on the ground, reported citing medical officials in Nareb that a child was killed, although the cause of death was not clear.

The Local Coordination Committees, another activist group, also reported the alleged chemical attack in Saraqeb.

There was no mention of the attacks in Syrian state media.

Forces loyal to Assad have blamed opposition fighters for such attacks.

The latest suspected attack comes a day after the Organisation for the Prohibition of Chemical Weapons, which has links to the UN and the support of 190 member states, said it was ready to investigate multiple allegations of chlorine attacks in recent months.

The Syrian government would need to approve the group's visit.

First-hand accounts

Last month, a closed-door meeting of representatives from the UN Security Council heard first-hand accounts from Syrian doctors of alleged chemical weapons attacks in March in the village of Sarmin, which is also in Idlib province.

The US, along with Britain and France, accuse Assad's government of using chlorine gas against civilians as his is the only force using helicopters, but Russia maintains there is no firm evidence that the government is responsible.

The 15 envoys attending the UN meeting heard a report by a Syrian doctor who said that he treated the victims of the Sarmin attack, which killed six people from the same family; three children, their parents and grandmother.

Dr Zaher Sahloul, of the Syrian American Medical Society, told Al Jazeera that the scene was "very traumatic".

"Doctors and nurses were struggling to treat them. They all had respiratory symptoms, they had coughs, they had wheezing, some of them had fluid in their lungs," Sahloul said.

Rights group Amnesty International said the alleged chlorine gas attack in Sarmin was evidence of new war crimes by the Syrian government, and should be referred to the International Criminal Court "as a matter of urgency".

"These horrific attacks that resulted in civilians, including small children, suffering excruciating deaths, are yet more evidence that the Syrian government forces are committing war crimes with impunity," said Philip Luther, Middle East and North Africa programme director at Amnesty International.

Chlorine was first introduced as a chemical weapon in World War I with disastrous effects as gas masks were not widely available.

Most nations banned its use in war in the Geneva Protocol of 1925.