Britain struck Syria with air-launched cruise missiles on Saturday to cripple President Bashar al-Assad's chemical weapons facilities and prevent what Prime Minister Theresa May cast as a global slide towards their greater use.
Speaking in Downing Street, May said the military strikes should be a "warning to Russia" before holding the Syrian government accountable for the chemical attack.
Syrian state media slammed Western strikes on Saturday as illegal and "doomed to fail", after the US, France, and Britain launched a joint operation against the Damascus government.
"While this action is specifically about deterring the Syrian regime, it will also send a clear signal to anyone else who believes they can use chemical weapons with impunity", May said.
Asked why she had proceeded without consulting parliament, May cited operational security consideration. The decision followed discussions with her advisers about the legality of the military action, she told reporters.
"This legally questionable action risks escalating further.an already devastating conflict", he said, adding that May should have sought parliamentary approval.More news: Qatari Emir to skip Arab summit in Saudi Arabia
May reiterated that Britain made use of every possible diplomatic means to stop the use of chemical weapons, but there was no response and "so there is no practicable alternative to the use of force to degrade and deter the use of chemical weapons by the Syrian regime".
"The three leaders agreed that the military strikes taken against the Syrian Regime's chemical weapons sites had been a success", a No 10 spokesman said.
She repeated that it was about sending a strong message about chemical attacks.
Russia, which intervened in the war in 2015 to back Assad, has denied there was a chemical attack and has accused Britain of helping to stage the Douma incident to stoke anti-Russian hysteria.
May called the claim "grotesque and absurd".
Meanwhile, Britain's main opposition Labour leader Jeremy Corbyn described the US -led military strikes on Syria jointly staged by three leading Western countries as a "legally questionable action", saying that the British government should "not taking instructions from Washington".More news: Guardiola charged with improper conduct after Liverpool defeat
"Since this was blocked by Russian Federation there was no other alternative than to react the way they reacted at this time".
DUP leader Arlene Foster said she had spoken to May and the airstrikes were "limited but proportionate and justified".
"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 said.
"It was both right and legal to take military action together with our closest allies to alleviate further humanitarian suffering by degrading the Syrian regime's chemical weapons capability", May said.
The Henry Jackson Society foreign policy think-tank said inaction would have been "grossly irresponsible".
"The strikes would also have been planned to minimise any chance of causing Russian casualties on the ground, lest that widen the conflict", he told AFP.More news: Russia likely to call United Nations meeting over Syria attack, says Russian lawmaker
Executive director Alan Mendoza added: "This can not be a token effort to demonstrate our outrage".