US President Donald Trump says he will decide on a response to a suspected chemical weapons attack on a rebel-held city in Syria.
The attack in Douma, the last rebel-held city in Syria, killed dozens of people on Saturday local time. Syrian government forces had launched an air and ground assault the city in the eastern Ghouta district on Friday.
Medical relief group, The Union of Medical Care Organizations (UOSSM), said at least 60 people had been killed by the alleged chemical attack, including children, and more than 1000 injured at several sites in Douma.
"The numbers keep rising as relief workers struggle to gain access to the subterranean areas where gas has entered and hundreds of families had sought refuge," it said.
Reports have been varied, with some putting the death toll as high as 90 and others around 45. One video shared by activists showed bodies of about a dozen children, women and men, some with foam at the mouth. Reuters could not independently verify the reports.
A doctor in Ghouta quoted by UOSSM, Dr Muhammad, said patients were coughing blood, a symptom not seen in previous chemical attacks.
International agencies led by the Organisation for the Prohibition of Chemical Weapons are trying to establish exactly what happened. The Hague-based organisation said people were possibly gassed to death by a poisonous cocktail of sarin and chlorine.
Speaking at a Cabinet meeting, Trump promised a "major decision" over the next 24 to 48 hours and said one may come as soon as the end of the day.
Mr Trump said he was talking to military leaders to determine who was responsible for the attack, whether it was Syria, Russia or Iran, or all of them together. The Syrian government and its ally Russia have denied involvement in the attack.
"We'll be making that decision very quickly, probably by the end of the day. But we cannot allow atrocities like that," Mr Trump said.
"Nothing is off the table," he said, when asked if US military action was a possibility.
The stakes were further raised on Monday when unidentified war planes struck a Syrian air base near Homs, killing at least 14 people, including Iranian personnel. Syria and Russia accused Israel of carrying out the attack.
Israel, which has struck Syrian army locations many times in the course of its neighbour's seven-year civil war, has neither confirmed nor denied mounting the raid.
Israeli officials said the Tiyas, or T-4, air base was being used by troops from Iran and that Israel would not accept such a presence in Syria by its arch foe.
Global community considers responses to attack
The United Nations Security Council will meet on Monday following requests by several members to discuss the situation, including both the US and Russia.
The Russian military said on Monday it had found no traces of a chemical attack in the Syrian town of Douma, Interfax news agency reported.
The defence ministry said Russian military medics had examined patients in a hospital in Douma and found no symptoms of chemical poisoning, according to the agency.
Senior officials in the Trump administration were also set to meet at the White House.
Britain said it was working with its allies to agree a joint response to attack.
"If there is clear verified evidence of the use of chemical weapons and a proposal for action where the UK would be useful, then we will look at the range of options," Prime Minister Theresa May's spokesman said.
France said it would work closely with the United States on a response to the suspected chemical attack. Both countries agreed responsibility for the strike must be established.
French President Emmanuel Macron, who spoke to Mr Trump by phone on Sunday, had issued repeated warnings previously that France would strike if proof of lethal chemical attacks were established.
US government sources say the administration had not yet conclusively determined who carried out the attack but its initial assessment suggested a nerve agent was used.
US Defense Secretary Jim Mattis also said earlier he would not rule out military action such as air strikes if blame was proven. He accused Russia of falling short on its obligations to ensure that Syria abandoned its chemical weapons capabilities.
"The first thing we have to look at is why are chemical weapons still being used at all when Russia was the framework guarantor of removing all the chemical weapons," Mattis told reporters.
The UN human rights chief Zeid Ra'ad al-Hussein lambasted the Security Council for offering what he called only "feeble condemnations" of a possible chemical attack in Syria.
He said the failure to react more strongly could have dire consequences for decades, and despite powerful states being directly involved in the conflict in Syria, and they had as yet completely failed to halt the ominous regression towards a chemical weapons free-for-all.
There have been at least 35 chemical weapons attacks in Syria since 2013, and the parties to the conflict and their supporters had pushed the door wide open to the use of the banned weapons, he said.
Mr al-Hussein said the Security Council's collective shrug to yet another possible use of "one of the most ghastly weapons ever devised by man" was incredibly dangerous.