Rescuers and medics say at least 70 people have died in Syria in a suspected gas attack in Douma, the last rebel-held town in Eastern Ghouta.
Volunteer rescue force the White Helmets tweeted graphic images showing several bodies in a basement. It said the death toll is likely to rise.
There has been no independent verification of the reports.
Syria’s government has called the allegations of a chemical attack a “fabrication”.
The US state department said it is monitoring the “very disturbing” reports, and that Russia – which is fighting alongside the Syrian government – should be held responsible if deadly chemicals had been used.
‘We will stay until the end’: A doctor’s battle in Eastern Ghouta
“The regime’s history of using chemical weapons against its own people in not in dispute,” said the state department.
It said it believed at least 40 people died, but that the toll may be far higher.
What do we know about the alleged gas attack?
Several medical, monitoring and activist groups reported details of a chemical attack, but figures vary and what happened is still becoming clear.
The pro-opposition Ghouta Media Center tweeted that more that 75 people had “suffocated” while a further thousand people had suffered the effects of the alleged gas attack.
It blamed a barrel bomb allegedly dropped by a helicopter which it said contained sarin, a toxic nerve agent.
The Union of Medical Relief Organizations, a US-based charity that works with Syrian hospitals, told the BBC the Damascus Rural Specialty Hospital had confirmed 70 deaths.
Pro-government forces are fighting to drive rebels out of Douma
A spokeswoman said reports on the ground suggested a much higher number of around 180 dead, but that it was hard to reach victims due to continuous shelling and the time of night.
She added 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.
As the allegations emerged, Syria’s state news service Sana said the reports were invented by the Jaish al-Islam rebels who remain in control in Douma.
“Jaish al-Islam terrorists are in a state of collapse and their media outlets are [making] chemical attack fabrications in an exposed and failed attempt to obstruct advances by the Syrian Arab army,” state media said.
Has the Syrian government used gas before?
In August 2013, rockets containing the nerve agent sarin were fired at rebel-held areas of Eastern Ghouta, killing hundreds of people.
A UN mission confirmed the use of sarin, but it was not asked to state who was responsible. Western powers said only Syrian government forces could have carried out the attack.
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 (OPCW) held the Syrian government responsible.
About 80 people died and many more were injured in the attack on Khan Sheikhoun
Syria government behind 2017 Sarin attack – UN
Activists, medics and the US say Syrian government forces dropped bombs containing toxic chlorine gas on rebel-held towns in early 2018.
The joint UN-OPCW mission is investigating the reports. It has previously found that government forces have used chlorine as a weapon at least three times during the seven-year civil war.
What’s happening in Douma?
Douma is the last rebel-held town in Syria’s Eastern Ghouta region, and is under siege from Russian-backed Syrian government forces.
An intense aerial and ground assault was launched on Friday after talks between Moscow and the rebels broke down.
Before negotiations failed, Jaish al-Islam had been trying to secure a deal that would let its members stay in Douma as a local security force.
The UK-based Syrian Observatory for Human Rights, a monitoring group, said government air strikes had killed 40 civilians on Friday, and a further 30 on Saturday.
State media said six civilians had also died in rebel shelling of the capital Damascus, with 38 injured. Jaish al-Islam denied it was responsible.