The US, UK and France have bombed multiple government targets in Syria in an early morning operation targeting alleged chemical weapons sites.
The strikes are in response to a suspected chemical attack on the Syrian town of Douma last week.
Explosions hit the capital, Damascus, as well as two locations near the city of Homs, the Pentagon said.
Russia's ambassador to the US responded by saying the attack on its ally "will not be left without consequences".
Statement by the Ambassador Antonov on the strikes on #Syria:— Russia in USA (@RusEmbUSA) April 14, 2018
A pre-designed scenario is being implemented. Again, we are being threatened. We warned that such actions will not be left without consequences.
All responsibility for them rests with Washington, London and Paris. pic.twitter.com/QEmWEffUzx
"A combined operation with the armed forces of France and the United Kingdom is now under way," US President Donald Trump said in an address to the nation from the White House at about 21:00 local time.
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.
A 'one-time shot'
At a Pentagon briefing shortly after Mr Trump's announcement, Gen 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 and an important command post, also near Homs
Syrian state television said government forces had shot down more than a dozen missiles, and claimed only the research facility in Damascus had been damaged.
Three civilians had been injured in Homs, it said.
US Secretary of Defence James Mattis told reporters there were no reports of US losses in the operation.
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 confirmed the wave of strikes had ended.
UK strikes in Homs
Gen Dunford said the US had specifically identified targets that would "mitigate" the risk of Russian casualties. But the Pentagon said that Russia - which has forces on the ground in Syria in support of the government - had not been given advance notice of the targets.
UK Prime Minister Theresa May confirmed British involvement, saying there was "no practicable alternative to the use of force".
But she also said the strikes were not about "regime change".
UK strikes carried out by four 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, according to the UK ministry of defence.
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".
Syria has denied carrying out the Douma attack and its ally, Russia, had warned that Western military strikes would risk starting a war.
The Organisation for the Prohibition of Chemical Weapons (OPCW) has dispatched a fact-finding mission to the site of the alleged attack in Syria. Investigators were due to start their probe later on Saturday.
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.
Double the missiles
A US official told Reuters news agency that Tomahawk cruise missiles were being used against multiple locations in Syria.
Secretary Mattis 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.
The strikes were ordered "on targets associated with the chemical weapons capabilities" of the Syrian government, Mr Trump said.
The US president said the purpose was "to establish a strong deterrent against the production, spread and use of chemical weapons".
"These are not the actions of a man, they are the crimes of a monster instead," he said of Syrian President Bashar al-Assad.
British-based monitoring group, the Syrian Observatory for Human Rights, suggested that more targets than the three listed by the Pentagon had been hit.
It said western forces had struck "scientific research centres, several military bases, and the bases of the Republican Guard and Fourth Division in the capital Damascus and around it."
One Damascus resident told BBC News: "It was mayhem above us."
"I saw more than 20 anti-air missiles launched. They'd fly really high then start weaving across, like they were following their target.
"I didn't see the cruise missiles, but I saw some falling debris nearby."
What has the reaction been?
Reaction to the strikes was mixed among the international community.
The Nato Secretary General, Jen Stoltenberg, tweeted support for the western strikes, saying those who use chemical weapons "must be held accountable".
I support the actions by the US, UK and France against the #Syrian regime's chemical weapons facilities and capabilities. #NATO considers the use of chemical weapons unacceptable. Those responsible must be held accountable. https://t.co/HPQ4YpTT7Y— Jens Stoltenberg (@jensstoltenberg) April 14, 2018
Canadian Prime Minister Justin Trudeau also expressed his nation's support.
John Mc Cain, Chairman of the Senate Armed Services Committee and a top Republican who is often critical of the president, applauded Mr Trump for taking military action.
Across the aisle, however, some Democrats said that President Trump should come before Congress and receive authorisation for the use of military force.
Meanwhile, the 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."