The United Kingdom, United States and France have launched "precision strikes" in Syria in response to a suspected chemical weapons attack in the country.
US president Donald Trump announced the "combined operation" on Friday night, as Prime Minister Theresa May said there was "no practicable alternative to the use of force".
Both leaders denounced the "cruel and abhorrent" attack by the Syrian regime on the town of Douma last Saturday.
Mrs May said "every possible diplomatic channel" had been explored before authorising the strikes, adding that it was not a decision she had taken lightly.
She said: "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.
"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."
Mr Trump said the Syrian regime's use of chemical weapons on Douma was a "significant escalation in a pattern of chemical weapons use by that very terrible regime".