Theresa May will speak to Donald Trump and lead a meeting of the National Security Council to discuss the suspected chemical attack in Syria.
The United States is looking to the UK and France for support as it finalises its response to the assault on the rebel-held town of Douma.
President Trump and his French counterpart Emmanuel Macron have already agreed to coordinate a "strong, joint response" after talks by telephone.
Mrs May, who spoke to President Macron on Tuesday morning, sidestepped questions about whether Britain would be involved in military action during a visit to Cambridge.
She said: "This attack that took place in Duma is a barbaric attack.
"Obviously we are working urgently with our allies and partners to assess what has happened on the ground.
"If this is the responsibility of Assad's regime in Syria then it's yet another example of the brutality and brazen disregard for their people that they show.
"I spoke this morning to President (Emmanuel) Macron, I will be speaking later today with President (Donald) Trump and the National Security Council meets regularly, and I'll be chairing a meeting of the National Security Council later today, and we'll be working with our allies as I say, crucially, to make an assessment of what has happened on the ground."
The National Security Council brings together relevant intelligence and defence chiefs and Cabinet ministers for top level briefings.
President Trump has said an apparent poison gas attack in Syria will be "met forcefully" and held talks with his military leaders in Washington on Monday night.
Mr Trump did not give a time frame for any retaliatory action, but said the US could not stand by as such atrocities take place because "we are able to stop it".