"This legally questionable action risks escalating further.an already devastating conflict", he said, adding that May should have sought parliamentary approval.
May referred specifically to last month's nerve agent attack on former spy Sergei Skripal and his daughter in the southern English cathedral city of Salisbury that she has blamed on Russian Federation.
Russian Federation circulated a draft resolution calling for condemnation of the military action, but Britain´s ambassador said the strikes were "both right and legal" to alleviate humanitarian suffering in Syria.
Shortly after Trump began a White House address to announce the action, large explosions were heard in the Syrian capital Damascus, signalling a new chapter in a brutal seven-year-old civil war.
"Initial indications are that the precision of the Storm Shadow weapons and meticulous target planning have resulted in a successful attack", it said.
Asked if the action had also been a warning to Russia, Mrs May said: "The action that took place last night was an action which was focused on degrading and deterring the operational capability and the willingness of the Syrian regime to continue to use chemical weapons". But she declined to give any signal about the future of Assad.
This is the first time as Prime Minister that I have had to take the decision to commit our armed forces in combat - and it is not a decision I have taken lightly.
British jets fired missiles at a Syrian military base suspected of holding chemical weapons ingredients on Saturday in Britain's first military action against President Bashar al-Assad's regime. Moscow has denied any involvement. She said it was clear his regime was responsible for the attack in the rebel-held town of Douma last Saturday which left 75 people dead.More news: Kathua Rape-Murder Case: What we know so far
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French President Emmanuel Macron said the strikes had been limited so far to Syria´s chemical weapons facilities.
The small Northern Irish political party that props up her government said May was justified in taking such action though it said wider intervention in Syria would be counter-productive.
By launching strikes without prior approval from parliament, Prime Minister Theresa May dispensed with a non-binding constitutional convention dating back to the 2003 invasion of Iraq, but said speed was essential and that military action was in the national interest.
"Bombs won't save lives or bring about peace", he said.
Shortly after the military strikes were launched, Scotland's first minister Nicola Sturgeon said United Kingdom foreign policy should be set by Parliament and not Donald Trump after the U.S., United Kingdom and France bombed targets in Syria.
"Britain should be playing a leadership role to bring about a ceasefire in the conflict, not taking instructions from Washington and putting British military personnel in harm's way", Corbyn said.
Often when the British government decides on military action, the opposition offers its full support.
British Prime Minister Theresa May described the strike as "limited and targeted". No debate in Parliament about the action or its potential consequences.