President Donald Trump said the air strikes were in response to the "evil and despicable" chemical attack on April 7 which left "mothers and fathers, infants and children, thrashing in pain and gasping for air".
He warned that intervention would lead to a proxy war with Russian Federation which would be "not only risky to Britain, but the entire world".
French President Emmanuel Macron said the strikes had been limited so far to Syria's chemical weapons facilities.
The joint attack by the United States, Britain, and France, which began at 3:55 a.m. local time (0055 GMT), launched some 110 missiles targeting Syrian military sites in the Syrian capital Damascus and elsewhere, Syria's state news agency SANA said, citing the military.
The Ministry of Defence (MoD) said that four RAF Tornado jets were deployed to launch missiles at a Syrian military facility.
She added that she would address parliament on Monday.
May has repeatedly said that the missile strike on Syria was not about "regime change". Western governments, including Britain, blamed the government of Bashar al-Assad, a Russian ally, for the attack.
May said that the objective of the strike was to "degrade and deter" the Syrian regime in its use of chemical weapons.
Britain's defense ministry said "very careful scientific analysis" had been applied to maximize the destruction of stockpiled chemicals while minimizing any risk of contamination to surrounding areas.
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Mr Putin and Mr Macron instructed their foreign and defence ministers to maintain close contact to "de-escalate the situation", the Kremlin said.
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Williamson told BBC radio that all British crews returned safely and early indications were that the strikes had been "highly successful".
The Syrian Regime has a history of using chemical weapons against its own people in the most cruel and abhorrent way.
May held an emergency cabinet meeting to discuss possible action on Thursday and there had been calls for the British parliament to be consulted before any air strikes.
"We have seen a regime appearing to think they can use chemical weapons with impunity".
British lawmakers voted down taking military action against Damascus in 2013, in what was widely viewed as an assertion of parliamentary sovereignty on the use of force.
For his part, the leader of the UK Independence Party (UKIP), Gerard Batten, opposed the British military action in Syria.
But they backed action in Iraq the following year, and again in Syria in 2015, strictly limiting strikes to targets of the Islamic State group.
Scottish First Minister Nicola Sturgeon also said the strikes risked "dangerous escalation".