The White House says it is “confident” Bashar al-Assad’s government was behind an apparent chemical attack that killed at least 58 people in north-west Syria.
The Syrian Observatory for Human Rights reported that strikes on Khan Sheikhoun by Syrian government or Russian jets had caused many people to choke.
Later, aircraft fired rockets at local clinics treating survivors, medics and activists said.
Syria’s army denied the government had used any such weapons.
White House spokesman Sean Spicer condemned what he called “these heinous actions by the Bashar al-Assad regime”, and said it was in the best interests of the Syrian people for President Assad to leave.
He was joined in his condemnation by the UK, United Nations and France, among others.
If confirmed, it would be one of the deadliest chemical attacks in Syria’s civil war.
What witnesses saw
The warplanes are reported to have attacked rebel-held Khan Sheikhoun, about 50km (30 miles) south of the city of Idlib, early on Tuesday, when many people were asleep.
- Hussein Kayal, a photographer for the pro-opposition Edlib Media Center (EMC), told the Associated Press that he was awoken by the sound of an explosion at about 06:30 (03:30 GMT)
- When he reached the scene, there was no smell, he said, and he found people lying on the floor, unable to move and with constricted pupils
- Mohammed Rasoul, the head of a charity ambulance service in Idlib, told the BBC his medics had found people, many of them children, choking in the street
- The UK-based monitoring group, the Syrian Observatory for Human Rights (SOHR), quoted medics who had treated people for fainting, vomiting and foaming at the mouth
- An AFP news agency journalist saw a young girl, a woman and two elderly people dead at a hospital, all with foam still visible around their mouths
- The journalist also reported that the same facility was hit by a rocket on Tuesday afternoon, bringing down rubble on top of doctors treating the injured.
The source of the projectile was not clear, but the EMC and the opposition Local Co-ordination Committees (LCC) network said warplanes had targeted several clinics.
The spectre of nerve agents in Syria – again
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Pro-government journalists later cited military sources as saying there had been an explosion at an al-Qaeda chemical weapons factory in Khan Sheikhoun that was caused either by an air strike or an accident.
The defence ministry of Russia, President Assad’s ally, insisted it had not carried out any air strikes in the vicinity.
How many victims are there?
The SOHR put the death toll at 58, including 11 children, but Mr Rasoul reported that 67 people had been killed and 300 injured.
The pro-opposition Step news agency said 100 had died.
One aid agency, the Union of Medical Care and Relief Organisations (UOSSM), put the death toll at more than 100.
The EMC said it had stopped counting the victims because there were so many.
The SOHR said it was unable to say what exactly was dropped.
However, the EMC and LCC said it was believed to be the nerve agent Sarin, which is highly toxic and considered 20 times as deadly as cyanide.
Chemical weapons expert Dan Kaszeta said that determining whether Sarin was involved simply by examining video clips is problematic.
He added that Tuesday’s attack could have been the result of one of any number of chemical agents as they tend to “behave the same in terms of their physiological effects on the human body”.
Sarin is almost impossible to detect because it is a clear, colourless and tasteless liquid that has no odour in its purest form.