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 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".
Russian talks with the rebels, the Jaish al-Islam group, broke down last week and fighting resumed.
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
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?
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.
Could the US take military action?
In April 2017, more than 80 people died in a Sarin attack on the opposition-held town of Khan Sheikhoun, and a joint inquiry by the UN and the Organization for the Prohibition of Chemical Weapons found the Syrian government responsible.
In response, President Trump ordered a cruise missile attack on a Syrian air base.
Asked if America might strike again after Saturday's reported attack, White House homeland security adviser Tom Bossert told ABC television: "I wouldn't take anything off the table."
French President Emmanuel Macron has also threatened to strike Syria if the government uses chemical weapons against civilians.
Before Mr Trump tweeted, the US state department said the attack, if confirmed, called for an "immediate response by the international community".
UK Foreign Secretary Boris Johnson said on Sunday that the suspected chemical attack must be investigated urgently.
"We are in close touch with our allies following these latest reports," he said. "Those responsible for the use of chemical weapons have lost all moral integrity and must be held to account."
What do Syria and Russia say?
Syrian state media accused "terrorist" media of fabricating reports about a chemical attack in order to hinder the advance of government forces through the Eastern Ghouta.
Russia's foreign ministry said reports of a chemical attack by Syrian forces on Douma had been "planted" in order to create a pretext for a possible military intervention in Syria.
"The purpose of these mendacious conjectures, which are without any basis, is to shield the terrorists and the irreconcilable radical opposition, which rejects a political settlement, while at the same time trying to justify possible external use of force," it said.
Russia launched its military operation in Syria in September 2015, saying it had been asked to intervene in the civil war by the Syrian government.
In seven years, the war has left more than 400,000 people dead or missing presumed dead, while more than half the population have been driven from their homes.