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.
The attack caused many people to choke or faint, and some had foam coming from their mouths, the Britain-based Syrian Observatory for Human Rights said, citing medical sources who described the symptoms as possible signs of a gas attack.
Locals said the attack began in the early morning, when they heard planes in the sky followed by a series of loud explosions after which people very quickly began to show symptoms.
They said they could not identify the planes.
Both Syrian and Russian jets have bombed the area before.
Russia’s defence ministry denied it was responsible, telling the State-run RIA news agency that it carried out no bombing runs in the area on Tuesday.
The Syrian government has repeatedly denied using such weapons and the Syrian army could not immediately be reached for comment.
Opposition activtists and the AFP news agency, citing one of its journalists on the scene, later said a rocket had slammed into a hospital where the victims were being treated, bringing rubble down on 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.
Alan Fisher, reporting from Beirut, said locals on the ground expected that the number of dead would increase and that many of the wounded were children.
“There were people fainting, they were vomiting, they were foaming at the mouth,” Fisher said.
“In that situation, the treatment tends to be to try and strip people off, to get the chemicals away from their bodies, to hose them down as quickly as possible.
But even then some of the pictures that have been posted on social media in the last couple of hours show very young people struggling for breath, many people dead where they fell.”
Fisher reported that hospitals in the area were overwhelmed with the scale of the apparent attack and that footage showed them struggling to cope with the number of victims.
The Edlib Media Centre (EMC), a pro-opposition group, posted images that were widely shared on social media, showing many people being treated by medics and what appeared to be dead bodies, many of them children.
The national opposition in Syria called for an immediate United Nations investigation and for the UN Security Council to hold an emergency meeting and condemn those behind the attack.
The French government also called on the Security Council to meet, and Britain called for an investigation.
“A new and particularly serious chemical attack took place this morning in Idlib province. The first information suggests a large number of victims, including children.
I condemn this disgusting act,” France’s foreign minister, Jean-Marc Ayrault, said in a statement.
“In the face of such serious actions that threaten international security, I ask for everyone not to shirk their responsibilities,” he added.
Turkey’s President Recep Tayyip Erdogan told Russian President Vladimir Putin that the “inhuman” attack could endanger peace talks, AFP reported, citing sources.
On Sunday, suspected Russian fighter jets bombed a hospital in a city in Idlib, wounding several people, a rescue group said.
At least ten people were wounded when three air raids 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, 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.