Britain's Ministry of Defence says strikes were aimed at a military facility where the Syrian government was assessed to have stockpiled chemicals.
Four Tornado jets carried out the attack using Storm Shadow missiles on a military facility west of Homs, some distance from known concentrations of civilian habitation, it said.
"Very careful scientific analysis was applied to determine where best to target the Storm Shadows to maximise the destruction of the stockpiled chemicals and to minimise any risks of contamination to the surrounding area," the Ministry of Defence said in a statement.
UK Prime Minister Theresa May confirmed British involvement, saying there was "no practicable alternative to the use of force". But she also said the strikes were not about "regime change".
It was a limited and targeted strike that did not further escalate tensions in the region, and that did everything possible to prevent civilian casualties, she said.
Mrs May said while the strike was targeted at Syria, it sent a message to anyone who used chemical weapons. Britain has accused Russia of being behind a nerve agent attack on a former Russian spy in England last month, a charge Moscow has rejected.
French President Emmanuel Macron also confirmed his country's participation in the operation.
He said the strikes were aimed at preventing "Syrian regime impunity".
Le samedi 7 avril 2018, à Douma, des dizaines d’hommes, de femmes et d’enfants ont été massacrés à l’arme chimique.— Emmanuel Macron (@EmmanuelMacron) April 14, 2018
La ligne rouge a été franchie.
J’ai donc ordonné aux forces armées françaises d’intervenir. https://t.co/Vt9LcFcFzH pic.twitter.com/Dc726PHfAR
The text reads: "Saturday 7 April 2018, in Douma, dozens of men, women and children were killed using chemical weapons. A red line has been crossed. I therefore ordered French armed forces to intervene."
The Russian Ambassador to the United States, Anatoly Antonov, said in a statement that Russia had warned that "such actions will not be left without consequences"
And he said insulting the Russian president was "unacceptable and inadmissible".
Mr Antonov said the United States had no right to blame other countries.
The Syrian Observatory for Human Rights said a total of three scientific research centres had been hit in the attacks - two in Damascus and one in the Homs area - in addition to military bases in Damascus.
The Observatory said all the bases and facilities struck in the attack had been evacuated by the Syrian government earlier this week.
- Reuters / BBC