US President Donald Trump has promised a "forceful" response to the alleged chemical attack in Syria, as Western leaders consider what action to take.
"We have a lot of options military," he told reporters. He added that a response would be decided "shortly".
Mr Trump said the US was getting some "good clarity" on who was responsible for the incident in Douma on Saturday.
Medical sources say dozens were killed in the alleged attack but exact numbers are impossible to verify.
Mr Trump also discussed the incident with French President Emmanuel Macron late on Monday, and both leaders expressed a desire for a "firm response", the Elysee Palace said.
The AFP news agency quoted French government spokesman Benjamin Griveaux as saying on Tuesday that "if a red line had been crossed, there will be a response", adding that intelligence shared by the two leaders "in theory confirms the use of chemical weapons".
In February Mr Macron threatened to strike Syria if proof emerged of the use of chemical weapons.
UK Prime Minister Theresa May has said she "utterly condemns" the "barbaric" incident, and called for backers of President Bashar al-Assad to be held to account.