Russia has requested a United Nations meeting in New York later in the day, but this has not yet been confirmed.
The White House says it is continuing to assess intelligence and talk to its allies on how to respond.
The delegation from the Organisation for the Prohibition of Chemical Weapons (OPCW) will start its investigations on Saturday, but, to ensure their safety, few details are expected to be released about their movements.
Why is the West considering military action?
The call for action comes after a suspected chemical attack on the rebel-held town of Douma in the Eastern Ghouta on Saturday, which killed dozens of people, according to opposition activists, rescue workers and medics.
The US ambassador to the UN, Nikki Hayley, told the network: "We definitely have enough proof, but now we just have to be thoughtful in our action."
French President Emmanuel Macron also said he had "proof" that the Syrian government had attacked Douma with chemical weapons, without giving further details.
In the UK, cabinet ministers agreed that it was "highly likely" the Assad regime was responsible for the alleged attack and said the use of chemical weapons must not "go unchallenged".
During a phone call late on Thursday, UK Prime Minister Theresa May and US President Donald Trump agreed on the need to deter chemical weapon use in Syria.
They agreed to "keep working closely" on the issue, Mrs May's office said in a statement.
What has Trump said about the attack?
On Sunday, the day after the attack, the US president said Russian President Vladimir Putin bore responsibility for the "atrocity" in rebel-held Douma, because of his support for the Syrian government.
Mr Trump, who has cancelled a planned trip abroad, has been canvassing support for strikes from the leaders of France and the UK.