The US, UK and France have bombed three government sites in Syria in an early morning operation targeting chemical weapons facilities, they say.
The move is a response to a suspected chemical attack on the town of Douma last week which killed dozens.
Russian President Vladimir Putin said he condemned the Western strikes “in the most serious way”.
Russia, Syria’s main ally, had threatened military retaliation if any Russian forces had been hit.
“The nations of Britain, France, and the United States of America have marshalled their righteous power against barbarism and brutality,” US President Donald Trump said in an address from the White House at about 21:00 local time (01:00 GMT).
“The purpose of our actions tonight is to establish a strong deterrent against the production, spread, and use of chemical weapons,” he said.
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.