Donald Trump has warned there will be a "big price to pay" following an alleged chemical weapons attack by the Syrian government in eastern Ghouta.
The US president blamed Russian president Vladimir Putin and Iran for backing Bashar al-Assad, following reports more than 40 civilians were killed by poison gas on Saturday.
"Many dead, including women and children, in mindless CHEMICAL attack in Syria," he said on Twitter. "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.
"Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!"
Mr Trump's outburst comes the same day top homeland security advisers said the US would not rule out launching a missile attack in response to the alleged use of a chemical weapon.
"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 the photos of the incident are "horrible".
At least 25 people were killed and more than 500 injured after the alleged attack in Douma, a town six miles northeast of the capital Damascus, according to the Union of Medical Care and Relief Organisations.
The White Helmets rescue organisation said 40 people, mostly women and children, had been killed.
The pro-opposition Ghouta Media Centre alleged a helicopter had dropped a barrel bomb containing sarin, and another organisation claimed that a hospital had been hit by a chlorine bomb.
Distressing images circulating on social media appeared to show people foaming at the mouth and nose.
Syrian state media denied government forces had launched any chemical attack, and said rebels in Douma were in a state of collapse and spreading false news.
The UN Security Council is likely to meet on Monday to discuss the alleged chemical attack the British Mission to the UN said on Twitter.
Nine out of 15 members requested the emergency meeting, the Mission said.
Russia's Foreign Ministry said on Sunday reports of a gas attack in Syria were "fake news" and any military action taken based on such "false and fabricated pretexts" could lead to "the gravest consequences".
"The goal of these false speculations, which are not substantiated by any facts, is to cover up terrorists and irreconcilable radical opposition, which opposes political settlement, and to simultaneously try to justify potential external military strikes," the ministry said, according to Russian news agency Tass.
"It is necessary to once again caution that military intervention under false and fabricated pretexts in Syria, where the Russian servicemen stay at the request of the legitimate government, is absolutely unacceptable and may trigger the gravest consequences."
Mr Trump went on to attack Barack Obama over the former president's failure to intervene militarily when the Syrian regime used chemical weapons in 2013.
"If President Obama had crossed his stated Red Line In The Sand, the Syrian disaster would have ended long ago!" he wrote on Twitter. "Animal Assad would have been history!"
Mr Trump's statement appeared to contradict tweets he sent five years ago, in which he urged Mr Obama to "not attack Syria", warning "very bad things will happen".
Foreign secretary Boris Johnson said reports of a poison gas attack were "truly horrific" and implored Russia not to obstruct an investigation by the Organisation for the Prohibition of Chemical Weapons.
"Should it be confirmed that the regime has used chemical weapons again, it would be yet another appalling example of the Assad regime’s brutality and blatant disregard for both the Syrian people and its legal obligations not to use chemical weapons," he said.
"We condemn the use of chemical weapons by anyone, anywhere. We are in close touch with our allies following these latest reports. Those responsible for the use of chemical weapons have lost all moral integrity and must be held to account."
On Friday, the Syrian government launched a fierce air and ground assault on Douma, the last rebel-held town in eastern Ghouta.
State TV showed thick clouds of smoke rising from the town, where Jaish al-Islam is holding out after insurgents in other parts of eastern Ghouta accepted offers of safe passage to rebel-held areas in the north of the country.
Six civilians were killed on Saturday in mortar shelling of residential neighbourhoods of Damascus, and 38 others were injured, Syrian state media reported, accusing Jaish al-Islam of the attacks. The rebel group's spokesman issued a statement denying the accusation
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