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.
Where was hit?
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.
Was anyone killed?
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.
Will the strikes continue?
In his earlier address, President Trump had said: "We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents."
But Secretary Mattis said that "right now, this is a one-time shot".
Gen Dunford said the US had communicated with Russia ahead of the strikes through the normal procedures of their "deconfliction" hotline, which is used to prevent accidental clashes in a war zone with multiple international players. There had been concerns that if the US strike had hit Russian military personnel on the ground, it would further escalate tension.
What role did the UK and France play?
According to the UK Ministry of Defence, strikes carried out by four RAF Tornado jets hit one of the targets mentioned by the Pentagon - a military site near the city of Homs which is believed to have housed precursor materials for chemical weapons.
Eight Storm Shadow cruise missiles were fired by the jets.
Prime Minister Theresa May said there was "no practicable alternative to the use of force".
But she also said the strikes were not about "regime change".
She later added that while the assessment of the strikes' results was ongoing, she was confident of their success.
French President Emmanuel Macron also confirmed his country's participation in the operation.
"Dozens of men, women and children were massacred with chemical weapons," he said of the Douma incident a week ago - adding that "the red line had been crossed".
Analysis: Will this time be different?
Jonathan Marcus, BBC defence correspondent
This attack was more significant than the US strike against a Syrian air base a little over a year ago, but at first sight seems more limited than President Trump's rhetoric may have suggested.
Last year some 59 missiles were fired. This time a little over double that number were used.
The strikes are over for now, but there was a clear warning that if the Assad regime resorts to chemical weapons again, then further strikes may well follow.
Care was taken, say the Americans, to avoid both Syrian and "foreign" - for that read Russian - casualties.
But the fundamental questions remain. Will President Assad be deterred?
Last year's US strike failed to change his behaviour. This time, will it be any different?
How has Syria responded?
Sana, Syria's official state news agency, called the Western action "a flagrant violation of international law".
"The American, French and British aggression against Syria will fail," it said.
Syria has repeatedly denied using chemical weapons.
The international Organisation for the Prohibition of Chemical Weapons (OPCW) had dispatched a fact-finding team to the site of the alleged attack in Douma and was set to start work on Saturday. The mission will still go ahead, said the OPCW after the strikes.
The Syrian presidency has tweeted a short video of Bashar al-Assad walking into his office at 09:00 local time with the caption: "Morning of steadfastness".
In his speech, President Trump said Mr Assad had committed "the crimes of a monster".
What has the worldwide reaction been?
Reaction to the strikes was mixed among the international community.
President Putin called them "an act of aggression".
The BBC's Kevin Connolly in Moscow said President Putin had articulated Russian anger, but Moscow's claim that none of its own military assets were threatened may suggest that the threat of escalation through some sort of Russian retaliation has now been substantially reduced.
German Chancellor Angela Merkel - who had ruled out joining the military action - said she supported the strikes as "necessary and appropriate".
Nato Secretary General Jens Stoltenberg tweeted support for the strikes, saying those who use chemical weapons "must be held accountable".
Nato said it would hold a special meeting on Saturday, where the US, UK and France would brief other member states.
US Senator John McCain applauded Mr Trump for taking military action. The leading Republican and former prisoner-of-war, who chairs the Senate Armed Services Committee, is often critical of the president.
Other US politicians argued that President Trump should come before Congress and receive authorisation for the use of military force.
As a Purple Heart Vietnam veteran, I put my life on the line for this country. Sad to hear @realDonaldTrump has chosen to bomb #Syria. America is on the brink of WWIII, have no clear agenda and put our military personnel lives at risk. #Damascus #PeaceForSyria— Senator Dick Black (@SenRichardBlack) April 14, 2018
Meanwhile, United Nations Secretary General Antonio Guterres warned UN members of their responsibilities.
"There's an obligation, particularly when dealing with matters of peace and security, to act consistently with the Charter of the United Nations and with international law in general," he said.
"I urge all member states to show restraint in these dangerous circumstances."