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 Douma 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”.
The president’s comments came after Moscow’s ambassador to the UN warned of the potential consequences to Western intervention in Syria.
Vassily Nebenzia said US attacks on Syria “could lead to grave repercussions” during heated exchanges at the UN Security Council.
US ambassador Nikki Haley accused Russia of having “the blood of Syrian children” on its hands after Mr Trump said that “nothing’s off the table” in dealing with the alleged outrage.
Mr Nebenzia dismissed claims the Syrian regime had used chemical weapons as “fake news” as he called for inspectors from the Organisation for the Prohibition of Chemical Weapons to fly to Syria on Tuesday to visit the site of the attack, which has left at least 40 people, including children, dead.
Foreign Secretary Boris Johnson said in February that Britain should consider joining military action against Assad’s regime if there is fresh “incontrovertible” evidence he has used chemical weapons against his own people.
Mr Johnson discussed the situation with acting US secretary of state John Sullivan twice on Monday.
The US State Department said Mr Johnson and his Washington counterpart had discussed “potential further steps the US and UK governments might take in coordination with other partners”.
The attack in Douma occurred late on Saturday amid a resumed offensive by Syrian government forces after the collapse of a truce with the Army of Islam rebel group.
Syrian opposition activists and rescuers said poison gas was used on the rebel-held town near the capital, an allegation strongly denied by the Assad government.
Families were reportedly found suffocated in their homes and shelters, with foam on their mouths.
Reports suggested more than 500 people, mostly women and children, were brought to medical centres with difficulty breathing, foaming at the mouth, and burning sensations in the eyes.
The attack comes almost exactly a year after a chemical atrocity in the northern Syrian town of Khan Sheikhoun killed dozens of people.
That attack prompted the US to launch several dozen Tomahawk cruise missiles at a Syrian air base.
Russia and Syria have blamed Israel for an attack on a Syrian military airport on Monday that reportedly killed at least 14 people.