US President Donald Trump says there would be a "big price to pay" for a chemical attack against a besieged rebel-held town in Syria where medical aid groups reported dozens of people were killed by poison gas.
The Syrian state denied government forces had launched any chemical attack and Russia, President Bashar al-Assad's most powerful ally, called the reports bogus.
A joint statement by the medical relief organisation Syrian American Medical Society and the civil defence service, which operates in rebel-held areas, said 49 people had died in the attack late on Saturday in the town of Douma. Others put the toll even higher.
"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 to pay," Mr Trump wrote on Twitter on Sunday.
The Russian foreign ministry warned against any military action on the basis of "invented and fabricated excuses", saying this could lead to severe consequences.
The US launched a cruise missile strike on a Syrian air base last year in response to a sarin gas attack in northwestern Syria blamed on Mr Assad.
One of Mr Trump's top homeland security advisers said on Sunday the US would not rule out launching another missile attack.
"I wouldn't take anything off the table," White House homeland security and counterterrorism adviser Thomas Bossert said in an interview on ABC's This Week.
"We are looking into the attack at this point," he said, adding that the photos of the incident are "horrible".
In one video shared by activists, the lifeless bodies of around a dozen children, women and men, some of them with foam at the mouth, were seen. "Douma city, April 7... there is a strong smell here," a voice can be heard saying.
Reuters could not independently verify the reports.
Douma is in the eastern Ghouta region near Damascus. Mr Assad has won back control of nearly all of eastern Ghouta in a Russian-backed military campaign that began in February, leaving just Douma in rebel hands.
The Syrian government has repeatedly denied using chemical weapons during the conflict.
"We have received reports of many people killed and injured in Duma in the past 24 hours. We continue to be extremely concerned for people who remain in Douma who are being subjected to escalating hostilities," UN Office for the Coordination of Humanitarian Affairs spokeswoman Linda Tom said.