US, British and French forces launched air strikes on Syria in response to a suspected chemical gas attack that killed dozens of people, aiming to degrade its chemical weapons capabilities in the biggest intervention yet in the conflict by Western powers.
US President Donald Trump announced the military action from the White House, saying the three allies had 'marshalled their righteous power against barbarism and brutality'.
As he spoke, explosions rocked Damascus.
British Prime Minister Theresa May described the strike as 'limited and targeted' and said she had authorised the British action after intelligence indicated Syrian President Bashar al-Assad's government was responsible for an attack using chemical weapons in Douma last Saturday.
French President Emmanuel Macron said the strikes had been limited so far to Syria's chemical weapons facilities.
With more than 100 missiles fired from ships and manned aircraft, the allies struck three of Syria's main chemical weapons facilities, US Defence Secretary Jim Mattis and Joint Chiefs of Staff Chairman General Joseph Dunford said.
British defence secretary Gavin Williamson said the strikes had been 'highly successful'.
The targets included a Syrian centre in the greater Damascus area for the research, development, production and testing of chemical and biological weaponry as well as a chemical weapons storage facility near the city of Homs. A third target, also near Homs, contained both a chemical weapons equipment storage facility and a command post.
Mattis called the strikes a 'one time shot,' but Trump raised the prospect of further strikes if Assad's government again used chemical weapons.
'We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents,' the US president said in a televised address.
The Syrian conflict pits a complex myriad of parties against each other, with Russia and Iran giving Assad military and political help while fractured opposition forces have had varying levels of support at different times from the West, Arab states and Turkey.
The strikes risked raising tensions in an already combustible region, but appeared designed not to trigger a military response from Russia and Iran.
Nevertheless, Assad's government and Russia responded angrily.
'Again, we are being threatened. We warned that such actions will not be left without consequences,' Anatoly Antonov, Russia's ambassador to the United States, said on Twitter.
Syrian state media said the attack would fail and called it a 'flagrant violation of international law.'
Russia was likely to call for a meeting of the United Nations Security Council to discuss the air strikes, lawmaker Vladimir Dzhabarov, the deputy head of Russia's foreign affairs committee, was quoted by the RIA news agency as saying.
The strikes seemed unlikely to have much impact on the balance of power in Syria's seven-year-old civil war, in which Assad's government has steadily gained the upper hand against armed opponents since Russia intervened in 2015.
At least six loud explosions were heard in Damascus and smoke was seen rising over the city, a Reuters witness said. A second witness said the Barzah district of Damascus had been hit in the strikes. Barzah is the location of a major Syrian scientific research centre.
A senior official in a regional alliance that backs Damascus told Reuters the Syrian government and its allies had 'absorbed' the attack, and that targeted sites were evacuated days ago thanks to a warning from Russia.
Iran's Foreign Ministry condemned the US-led attacks and said Washington and its allies would bear responsibility for the consequences in the region and beyond, state media reported.
State-controlled Syrian TV said Syrian air defences shot down 13 missiles fired in the attack. The Russian defence ministry said none of the rockets launched had entered zones where Russian air defence systems are protecting military facilities in Tartus and Hmeimim.
The combined US, British and French assault appeared more intense than a similar strike Trump ordered almost exactly a year ago against a Syrian air base in retaliation for an earlier chemical weapons attack that Washington attributed to Assad.
At a Pentagon briefing, Dunford said the air strikes on Saturday were planned to minimize the risk of casualties among Russia's forces in Syria.
May said the missile strike was designed to minimise any civilian casualties and was not an attempt to change the Syrian government. Britain's defence ministry said initial indications were that the precision weapons and meticulous target planning had 'resulted in a successful attack'.
Mattis acknowledged that the United States conducted the air strikes only with conclusive evidence that chlorine gas was used in the April 7 attack in Syria. Evidence that the nerve agent sarin also was used is inconclusive, he said.
Allegations of Assad's chlorine use are frequent in Syria's conflict, raising questions about whether Washington had lowered the threshold for military action in Syria by now deciding to strike after a chlorine gas attack.
Mattis, who US officials said had earlier warned in internal debates that too large an attack would risk confrontation with Russia, described the strikes as a one-off to dissuade Assad from 'doing this again.'
But a US official familiar with the military planning said there could be more air strikes if the intelligence indicates that Assad has not stopped manufacturing, importing, storing or using chemical weapons, including weaponized chlorine.
The official acknowledged that could require a more sustained US air and naval presence in the region, as well as intensified satellite and other surveillance of Syria.
Trump has been leery of US military involvement in the Middle East, and is eager to withdraw roughly 2,000 troops who are in Syria as part of the battle against Islamic State militants.
The air strikes, however, risk dragging the United States further into Syria's civil war, particularly if Russia, Iran and Assad opt to retaliate.
'America does not seek an indefinite presence in Syria, under no circumstances,' Trump said in his eight-minute address.
'The purpose of our actions tonight is to establish a strong deterrent against the production, spread and use of chemical weapons,' he said.
The US president, who has tried to build good relations with Russian President Vladimir Putin, had sharply critical words for Russia and Iran over their support of Assad.
'To Iran and to Russia, I ask, what kind of a nation wants to be associated with the mass murder of innocent men, women and children?' Trump said.
Last year, the United States fired 59 Tomahawk cruise missiles from the guided missile destroyers USS Porter and the USS Ross that struck the Shayrat air base.
The targets of that strike included Syrian aircraft, aircraft shelters, petroleum and logistical storage facilities, ammunition supply bunkers, air defence systems and radar. At the time, the Pentagon said that a fifth of Syria's operational aircraft were either damaged or destroyed.
The US-led attack on Syria will be seen as limited if it is now over and there is no second round of strikes, said a senior official in the regional alliance that has supported Assad in the Syrian war.
'If it is finished, and there is no second round, it will be considered limited,' the official told Reuters.