US President Donald Trump has condemned a suspected chemical weapons attack on a rebel-held town in Syria that killed dozens of people and says he would make a decision on a response, probably within hours.
Speaking at a cabinet meeting on Monday, Mr Trump said he was talking to military leaders and would decide who was responsible for the attack - whether it was Russia, or Syrian President Bashar al-Assad's government, or Iran, or all of them together.
International bodies led by the Organisation for the Prohibition of Chemical Weapons were trying to establish exactly what happened on Saturday in Douma, a besieged town in eastern Ghouta near Damascus.
The Syrian government and its ally Russia have denied involvement in the attack.
US government sources said on Monday the administration had not yet conclusively determined whether the attack was carried out by Mr Assad's forces. Its initial assessment suggested that a nerve agent was used but further evidence was needed to determine what specifically it was, the sources said.
US Defense Secretary Jim Mattis said he would not rule out military action such as air strikes if blame was proven.
"I don't rule out anything right now," he told reporters in Washington.
Mr Mattis 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." he said.
Mr Trump said on Sunday after initial reports of an attack that there would be a "big price to pay".
A Syria medical relief group said at least 60 people had been killed and more than 1000 injured at several sites in Douma.
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-old civil war, has neither confirmed nor denied mounting the raid.
But 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.
The United Nations Security Council will meet on Monday following rival requests by Russia and the US to discuss the situation. Senior officials in the Trump administration were also to meet in the White House.
Britain said it was working with its allies to agree a joint response to the reported chemical attack on Douma.
"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 US on a response to the suspected chemical attack. Both countries agreed responsibility for the strike must be established.
President Emmanuel Macron, who spoke to Mr Trump by telephone on Sunday, had issued repeated warnings previously that France would strike if proof of lethal chemical attacks were established. But Paris stopped short of apportioning blame on Mr Assad's forces for Saturday's attack.