Air strikes were launched early on Saturday morning.
The US, UK and France have launched air strikes against what they allege are Syrian chemical weapons facilities, following a chemical weapons attack in a Damascus suburb a week ago.
More than 100 armaments, bombs and ship-launched cruise missiles, were deployed, the Pentagon said.
A chemical weapons storage facility west of Homs, another storage site and command post nearby and a scientific research centre in Damascus were the three official targets.
Four UK Tornado jets targeted one of the sites - the facility to the west of Homs.
Downing Street said the bombing is intended "to degrade the Syrian Regime’s chemical weapons capability and deter their use".
Last week's attack killed up to 75 people, Theresa May said, adding that a "significant body" of information and intelligence indicates Bashar al-Assad is responsible for the "cruel and abhorrent" war crime.
Explosions were reported in Damascus minutes after Trump publicly announced the strikes.
The wave of strikes is the most significant attack against President Bashar al-Assad's government by Western powers in seven years of Syria's civil war.
Theresa May said: "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.
"And 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.
"The speed with which we are acting is essential in co-operating with our partners to alleviate further humanitarian suffering and to maintain the vital security of our operations.
"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.
"I have done so because I judge this action to be in Britain’s national interest.
"We cannot allow the use of chemical weapons to become normalised – within Syria, on the streets of the UK, or anywhere else in our world.
"We would have preferred an alternative path. But on this occasion there is none.
"History teaches us that the international community must defend the global rules and standards that keep us all safe.
"That is what our country has always done. And what we will continue to do."
Jeremy Corbyn had previously called for a parliamentary vote on any military action but the Prime Minister went over his head.
https://twitter.com/jeremycorbyn/status/984834763725950978
After reaching an agreement with cabinet, May has taken executive action.
David Cameron sought parliamentary approval for a similar action in 2013 but lost and had to back down after Ed Miliband refused to provide his consent.
The Russian ambassador to Washington, Anatoly Antonov, issued a statement threatening "consequences".
"A pre-designed scenario is being implemented," Antonov’s statement said.
"Again, we are being threatened. We warned that such actions will not be left without consequences. All responsibility for them rests with Washington, London and Paris."
Earlier this week, Russian state television
instructed viewers to stockpile supplies and prepare for war.
