US President Donald Trump has lashed out at Syrian President Bashar al-Assad and his allies Russia and Iran over a suspected chemical attack, saying there will be a “big price to pay”.
Medical sources say dozens of people were killed in an attack on the rebel-held town of Douma on Saturday.
The UN Security Council may debate the attack on Monday after nine of its 15 members called for an urgent meeting.
Both Syria and Russia deny a chemical attack took place.
The European Union has called for an “immediate response by the international community”.
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At the Vatican, Pope Francis said: “Nothing, nothing can justify the use of such devices of extermination against defenceless people and populations.”
Mr Trump described President Assad as an “animal” in a series of tweets.
Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price…
— Donald J. Trump (@realDonaldTrump) April 8, 2018
….to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!
— Donald J. Trump (@realDonaldTrump) April 8, 2018
If President Obama had crossed his stated Red Line In The Sand, the Syrian disaster would have ended long ago! Animal Assad would have been history!
— Donald J. Trump (@realDonaldTrump) April 8, 2018
As details of the attack emerged, the Russian military reached a deal with the rebels controlling Douma, the Jaish al-Islam group, on a partial evacuation to end the fighting there.
Russian military police will be admitted to the town while the rebels will be allowed either to leave or stay with a guarantee they will not be pursued by Syrian security forces, local negotiators told Reuters news agency.
Syrian state media said earlier that a deal had been struck with Jaish al-Islam to allow them to leave Douma within 48 hours in return for them freeing prisoners.
Douma is the last rebel bastion in the Eastern Ghouta region outside the capital, Damascus, following a government offensive.
What do we know about the attack?
One video, recorded by rescue workers known as the White Helmets, shows a number of men, women and children lying lifeless inside a house, many with foam at their mouths.
Other unverified footage shows young children crying as they are treated in a makeshift medical unit.
However, it has not been possible to verify independently what actually happened, or the actual number of dead.
The Union of Medical Care and Relief Organizations, which runs medical facilities in the Eastern Ghouta, told BBC News that 70 deaths had been confirmed.
According to the US-based Syrian American Medical Society, at least 48 people died, showing “symptoms indicative of exposure to a chemical agent”.
More than 500 people were brought to medical centres with such symptoms, it said, quoting emergency services in Douma.
A doctor at an overwhelmed hospital in the rebel-controlled town told the BBC’s Joel Gunter he had panicked as his own children coughed from gas seeping into the basement where they were hiding.
A medical student described in horrifying detail how he had treated a dying man.
