US President Donald Trump on Monday condemned a suspected chemical weapons attack on a rebel-held town in Syria that killed dozens of people and said he would decide on a response probably by the end of the day.
Speaking at a Cabinet meeting, Trump said he was talking to military leaders and would decide who was responsible for what he called a "heinous attack" on innocent Syrians – whether it was Russia, Syrian President Bashar al-Assad's government, Iran, or all of them together.
"We'll be making that decision very quickly probably by the end of the day. But we cannot allow atrocities like that," Trump said. "Nothing is off the table," he said, when asked if US military action was a possibility.
Asked if there was any doubt over who was responsible for the attack, Trump said: "To me there's not much of a doubt but the generals will figure it out probably over the next 24 hours."
International bodies led by the Organization for the Prohibition of Chemical Weapons (OPCW) were also 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 that the administration's initial assessment suggested that a nerve agent was used but further evidence was needed to determine what specifically it was.
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.
Trump said on Sunday after initial reports of an attack that there would be a "big price to pay".