The US, Britain and France launched coordinated strikes against Syria's research, storage and military targets to "punish" the Bashar al-Assad regime for an apparent chemical attack in Douma that killed over 70 people. The military action was denounced by Damascus and its ally Moscow as a "failure" and "an act of aggression".
Western allies warned Syria on Saturday that they could launch further attacks if chemical weapons were used again. The strikes on Friday night was intended to show Western resolve in the face of what the leaders of the three nations called "persistent violations of international law".
US President Donald Trump via a tweet hailed the overnight military strike as "perfectly executed", adding: "Mission Accomplished".
The strikes targeted three facilities associated with Syria's chemical weapons arsenal, including a scientific research facility around Damascus, a chemical weapons storage facility around Homs alleged to be used for sarin gas and a nearby command post, said the Pentagon.
The Syrian Observatory for Human Rights said the Syrian Army's 4th Division and Republican Guard were among the targets. US aircraft including B-1 bombers, naval vessels and about 100 Tomahawk cruise missiles were used in the attack, the Pentagon said.
"The nations of Britain, France and the US have marshalled their righteous power against barbarism and brutality," Trump said in an address after the strike.
"The purpose of our actions is to establish a strong deterrent against the production, spread and use of chemical weapons...The wave of strikes is the most significant attack against Assad's government by Western powers in seven years of Syria's civil war," he said.
"We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents," he added.
In Syria, the Foreign Ministry described the missile strikes as "barbaric aggression". Assad said that it reflected the "failure" of Western powers to achieve their goals in Syria after the defeat of the foreign-backed militants.
The Syrian military claimed that its air defences shot down a majority of the 110 missiles launched at dawn by the Western allies and claimed only the research facility in Damascus had been damaged. Russia said that Syrian forces used older Soviet-made air defence systems to intercept incoming missiles.
Civilians and soldiers gathered in Ummayad Square in Damascus for a show of support, waving Syrian flags and dancing to songs that praised the Army.
Russian President Vladimir Putin slammed the US and its allies, saying that Washington was "increasingly exacerbating the humanitarian catastrophe in Syria" and called for an immediate UN Security Council meeting to discuss the "an act of aggression".
He said the US used a "staged chemical attack" against civilians to carry out the latest strike, adding that Russian military experts did not find any traces of chlorine gas or other poisonous substances in Douma.
Russian Foreign Ministry spokeswoman Maria Zakharova said that "the action against Syria came precisely at the moment when the country received a chance for a peaceful future".
Iran's Supreme Leader Ayatollah Ali Khamenei called the attack a "crime" but gave no indication of any planned Iranian response. Iran and its main proxy force in the region, Lebanon's Hezbollah, are both key allies of Syria.
British Prime Minister Theresa May and French President Emmanuel Macron confirmed their countries' involvement in the action with the former saying that "Syria had left the allies no choice".
Macron, talking about the Douma chemical attack, said: "Dozens of men, women and children were massacred with chemical weapons. The red line had been crossed."
"France and its partners will today resume their efforts at the UN to enable the creation of an international mechanism to establish responsibility, prevent impunity and obstruct any temptation on the part of the Syrian regime to repeat these acts."
Nato Secretary General Jen Stoltenberg tweeted that those who use chemical weapons "must be held accountable".
The EU said it "was supportive of all efforts aimed at preventing the use of chemical weapons".
German Chancellor Angela Merkel -- who had ruled out joining the military action -- said she supported the strikes as "necessary and appropriate".
UN Secretary General Antonio Guterres warned UN members of their responsibilities. "I urge all member states to show restraint in these dangerous circumstances."
China's Foreign Ministry urged negotiation and called for a "comprehensive and impartial" investigation into the allegation that chemical weapons were used.
Meanwhile, a fact-finding mission from the Organization for the Prohibition of Chemical Weapons was to begin investigating the episode on Saturday in Douma, which had been held by rebels before the suspected attack.