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.
At a Pentagon briefing shortly after Mr Trump's announcement, General Joseph Dunford listed three targets that had been struck:
- A scientific research facility in Damascus, allegedly connected to the production of chemical and biological weapons
- A chemical weapons storage facility west of Homs
- A chemical weapons equipment storage site and an important command post, also near Homs
Reuters news agency cites a pro-Assad militia commander saying other locations were hit, including various sites close to Damascus: a military base in the Dimas area; army depots in the eastern Qalamoun; the Kiswah area, where Iran is believed to have been building a base; and a site in the Qasyoun hills, plus a research centre in Masyaf, further north. These reports are unverified.
UK-based monitoring group the Syrian Observatory for Human Rights suggested more targets than the three listed by the Pentagon had been hit.
Russia said Syria had shot down 71 of 103 missiles fired.
According to a Russian defence ministry statement, "preliminary information" said there had been no casualties among the Syrian army or civilians.
There were initial reports that three civilians had been injured in Homs.
US Secretary of Defence James Mattis told journalists there were no reports of US losses in the operation.
He also said the scale of the strikes was about "double" what was launched in April 2017 after a chemical attack on the town of Khan Sheikhoun that killed more than 80 people. .