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”.
Scores of people are said to have been killed in an attack on the rebel-held town of Douma on Saturday.
The UK called for an urgent inquiry while Pope Francis said nothing could justify using chemical weapons.
Both Syria and Russia deny a chemical attack took place.
In a series of tweets, Mr Trump described President Assad as an “animal”.
The attack came after Russian talks with the rebels, the Jaish al-Islam group, broke down last week and both sides resumed fighting.
However, according to Syrian state media, a deal was struck with Jaish al-Islam on Sunday to allow them to leave Douma within 48 hours in return for them freeing prisoners. The rebel group has not commented.
Douma is the last rebel bastion in the Eastern Ghouta region outside the capital, Damascus, following a government offensive and other evacuation deals brokered by Russia.
What do we know about the attack?
Earlier, the US state department said the attack, if confirmed, called for an “immediate response by the international community”.
It has not been possible to verify independently what actually happened, or the actual number of dead.
The Union of Medical Relief Organizations, a US-based charity that works with Syrian hospitals, told the BBC 70 deaths had been confirmed by the Damascus Rural Specialty Hospital.
A spokeswoman said there were reports of people being treated for symptoms including convulsions and foaming of the mouth, consistent with nerve or mixed nerve and chlorine gas exposure.
However, the UK-based Syrian Observatory for Human Rights said 11 people had died of suffocation, adding that it could not say if chemical weapons had been used.
One video, recorded by rescue workers known as the White Helmets, shows a number of men, women and children lying lifeless inside a house, again, many with foam at their mouths.
Other unverified footage shows young children crying as they are treated in a makeshift medical unit.