Donald Trump said "precision strikes" targeted the country's chemical weapons programme
The US, UK and France have launched military strikes on Syria following the suspected chemical attack by the Assad regime.
Over 70 people died and a further 500 were injured in the incident in the town of Douma near Damascus on Saturday.
US 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.
British Prime Minister Theresa May confirmed she had ordered UK armed forces to conduct "co-ordinated and targeted strikes to degrade the Syrian regime's chemical weapons capability and deter their use".
In a statement, Mrs May said: "The Syrian regime has a history of using chemical weapons against its own people in the most cruel and abhorrent way.
"And a significant body of information including intelligence indicates the Syrian regime is responsible for this latest attack.
"This persistent pattern of behaviour must be stopped - not just to protect innocent people in Syria from the horrific deaths and casualties caused by chemical weapons but also because we cannot allow the erosion of the international norm that prevents the use of these weapons.
"We have sought to use every possible diplomatic channel to achieve this.
"But our efforts have been repeatedly thwarted. Even this week the Russians vetoed a Resolution at the UN Security Council which would have established an independent investigation into the Douma attack.
"So there is no practicable alternative to the use of force to degrade and deter the use of chemical weapons by the Syrian regime.
"This is not about intervening in a civil war. It is not about regime change."
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".
"We cannot tolerate the normalisation of the use of chemical weapons," he said in a statement.
The US Defence Secretary James Mattis said more details about the operation would be known on Saturday morning.