Donald Trump said air strikes against Syria are "underway" in a combined operation with the UK and France.
Giving a statement at the White House, the US President said they were in response to the alleged chemical weapons attack in the town of Douma last week,
He said: "This evil and despicable act left mothers and fathers and children thrashing in pain and gasping for air. The combined American, British and French response will integrate all instruments of our national power."
Prime Minister Theresa May said there was "no practicable alternative to the use of force" to deter the use of chemical weapons by the Syrian regime.
She added that "every possible diplomatic channel" had been sought before the decision to launch targeted strikes had been taken.
"This persistent pattern of behaviour must be stopped - not just to protect innocent people in Syria from the horrific deaths and casualties caused by chemical weapons but also because we cannot allow the erosion of the international norm that prevents the use of these weapons," she said. "This is not about intervening in a civil war. It is not about regime change.
"It is about a limited and targeted strike that does not further escalate tensions in the region and that does everything possible to prevent civilian casualties."
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