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 militarily," 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".
UK Prime Minister Theresa May said she had spoken to Mr Macron on Tuesday morning and would be speaking to Mr Trump later in the day. Mrs May did not answer directly when asked if Britain would join the US if it decided to take a military response, saying instead: "We believe that those responsible should be held to account."
The condemnation from Western leaders follows a tense meeting at the UN Security Council in which the US and Russia traded harsh words over the incident.
US envoy Nikki Haley has called for a vote on Tuesday on a draft resolution to set up a new inquiry into the use of chemical weapons in Syria.
But Russia says it cannot support the proposal because it contains "unacceptable elements".
Foreign Minister Sergei Lavrov said Moscow would submit its own draft resolution proposing that inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) visit Douma.
Russia wanted independent experts to be convinced that nothing had happened and would do everything to allow such a visit to take place, he added.
Russian officials said on Monday that Russian military specialists and staff from Syria's Red Crescent had found no evidence of an attack.
What happened on Saturday?
Syrian opposition activists, rescue workers and medics allege that bombs filled with toxic chemicals were dropped on Douma, in the Eastern Ghouta region, near the capital Damascus, by Syrian government forces.
The Syrian-American Medical Society said more than 500 people were brought to medical centres with symptoms "indicative of exposure to a chemical agent".
It said this included breathing difficulties, bluish skin, mouth foaming, corneal burns and "the emission of chlorine-like odour".
Neither the death toll nor what exactly occurred can be verified as the area is blocked off with access denied.
The estimates of how many people died in the suspected chemical attack range from 42 to more than 60 people, but medical groups say numbers could rise as rescue workers gain access to basements where hundreds of families had sought refuge from bombing.
The French representative at the UN Security Council said poison gas had deliberately been used as it could seep down to the basements.
The US, France and UK have led international condemnation of the alleged attack, with the Syrian government and its Russian backers denying any responsibility.
What happened at the UN?
Ms Haley said Russia - a Syrian military backer - had the "blood of Syrian children" on its hands and branded President Assad a "monster".
But Russia's UN Ambassador Vassily Nebenzia, presenting Russia's case that rebels in Douma had staged the event for their own ends, painted the incident and its fallout as part of a US-led effort to hurt Russia with a "broad arsenal of methods", including slander, insults and "hawkish rhetoric".
In an angry statement, he invited the OPCW to fly to Syria as soon as Tuesday, saying that Russian troops would escort them to the site of the alleged attack.
What is the wider context?
Tensions between Russia and the West have plunged to their worst level in decades, following the poisoning in March of an ex-spy in England that the UK blamed on Moscow, and alleged Russian interference in the 2016 US election.
The poisoning of Sergei Skripal and his daughter Yulia with what the British government says was a military-grade Novichok nerve agent of a type developed by Russia led to the mass expulsion of Russian diplomats by Western allies, to which Moscow responded in kind.