The US, UK and France have launched military strikes on Syria following the suspected chemical attack by the Assad regime.
President Donald Trump confirmed "precision strikes" had been ordered on targets associated with the Syrian government's chemical weapons programme.
In a televised address from the White House, Mr Trump said: "A short time ago, I ordered the United States Armed Forces to launch precision strikes on targets associated with the chemical weapons capabilities of Syrian dictator Bashar al Assad."
Mr Trump said the military action was a "combined operation" with the armed forces of the UK and France.
Prime Minister Theresa May confirmed she had ordered British armed forces to conduct "co-ordinated and targeted strikes to degrade the Syrian regime's chemical weapons capability and deter their use".
She said: "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."
The Ministry of Defence said British forces joined the allied strikes on Syria at 2am UK time.
Four RAF Tornado GR4s launched shadow missiles at a military facility, a former missile base, 15 miles west of Homs.
The MoD says the military action was "proportionate" and "specifically aimed at degrading the Assad regime's ability to use chemical weapons and deterring further such appalling acts".
French President Emmanuel Macron said that France had joined the US and Britain to target "the capacities of the Syrian regime to produce and use chemical weapons".
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"We cannot tolerate the normalisation of the use of chemical weapons," he said in a statement.