US President Donald Trump on Sunday condemned a reported chemical attack on a rebel-held town in Syria that killed dozens of people and warned there would be a "big price to pay".
At least 48 people died in the attack on Douma late on Saturday, activists and rescuers said.
Mr Trump said many women and children were among the victims in the "mindless chemical attack" and blamed Iran and Russia for supporting the regime of President Bashar Al Assad.
Iran on Sunday responded to allegations of a chemical attack by saying it was a "conspiracy" against its ally Mr Al Assad and a pretext for military action.
"Such allegations and accusations by the Americans and certain Western countries signal a new conspiracy against the Syrian government and people, and a pretext for military action," Iran's foreign ministry said.
Russia also denied that a chemical attack took place.
The alleged attack comes almost exactly a year since Mr Trump ordered US missile strikes on a Syrian airbase last April in response to a sarin gas attack on the north-western town of Khan Sheikhoun that killed at least 80 people.
Mr Trump's homeland security adviser, Thomas Bossert, noted the timing of the suspected chemical attack.
"This isn't just the United States. This is one of those issues on which every nation, all peoples, have all agreed and have agreed since World War Two, it's an unacceptable practice," Mr Bossert said.
Asked about the potential for an American missile strike in response, he said: "I wouldn't take anything off the table. These are horrible photos. We're looking into the attack at this point."
Douma is the last rebel-held town in the Eastern Ghouta region, east of Damascus, after nearly two months of bombardment by Syrian and Russian forces.
Syrian state media announced that the Jaish Al Islam rebel group holding Douma had resumed negotiations for a withdrawal on Sunday, and had later agreed to leave the town within 48 hours.
However, opposition negotiators told Reuters that the deal would allow Russian military police to enter the town and that those fighters who do not want to leave would be able remain without being targeted by Syrian security forces.
The alleged chemical attack came on the second day of a renewed government offensive on Douma after a lull of several days last week.
Syria's White Helmets, who act as first responders in rebel-held areas, said an attack took place late on Saturday using "poisonous chlorine gas".
In a joint statement with the Syrian American Medical Society (Sams), the White Helmets said more than 500 cases were brought to medical centres "with symptoms indicative of exposure to a chemical agent".
The group said six people died while under care and rescuers found 42 more people dead in their homes with signs of similar symptoms.
"The scene was horrifying. So many people were choking, so many people," White Helmets member Firas Al Doumi told Agence France-Presse.
"Most died immediately. The majority were women and children."
Footage posted online by the White Helmets showed victims with yellowed skin crumpled on the ground and foaming at the mouth. Children could be seen receiving treatment at hospitals, with shell-shocked medics holding up gas masks to motionless infants.
Sams said a chlorine bomb hit Douma hospital, and a second attack with "mixed agents" including nerve agents had hit a nearby residential building.
Sams operates 139 medical facilities in Syria where it supports 1,880 medical personnel, according to its website.
"We are contacting the UN and the US government and the European governments," Basel Termanini, the US-based vice president of Sams, told Reuters.
The US State Department said earlier that reports of mass casualties were "horrifying" and would, if confirmed, "demand an immediate response by the international community".
Britain's Foreign Office also called the reports "very concerning" and said "an urgent investigation is needed and the international community must respond. We call on the Assad regime and its backers, Russia and Iran, to stop the violence against innocent civilians.”
Russia, however, dismissed the reports. Maj Gen Yuri Yevtushenko, head of the Russian peace and reconciliation centre in Syria, was quoted by Interfax news service as saying: "We decidedly refute this information."
Syria: Air strikes kill 48 in Eastern Ghouta
Russia blamed for failure to stop Syrian chemical attacks
The Syrian government has repeatedly denied using chemical weapons during the conflict.
Mr Al Assad has won back control of nearly all of Eastern Ghouta in a Russian-backed military campaign that began in February. The offensive has been one of the deadliest of Syria's seven-year civil war, killing more than 1,600 civilians according to the Observatory.
Facing military defeat, rebel groups in other parts of Eastern Ghouta accepted safe passage to other opposition-held areas at the Turkish border. Until now, Jaish Al Islam has rejected that option, demanding it be allowed to stay in Douma.