The UN Security Council is meeting to discuss the US airstrikes against President Bashar al Assad's regime.
America fired dozens of missiles at a Syrian airbase overnight in retaliation for this week's chemical weapons attack on civilians, including babies.
President Donald Trump said the US military action was vital to deter future use of poison gas and called on other nations to join in to help "end the slaughter and bloodshed in Syria".
The strikes - the first US intervention against President Assad's forces in Syria's six-year war - drew immediate condemnation from staunch ally Russia, with Vladimir Putin calling the raids an "aggression against a sovereign state in violation of international law".
The Kremlin said the action would damage ties between Moscow and Washington and was a "serious obstacle" to the creation of an international coalition to fight terrorism. Moscow called for the UN Security Council meeting.
The Kremlin also said a "complex of measures" would be put in place to strengthen Syrian air defences and help "protect the most sensitive Syrian infrastructure facilities".
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Senator John McCain told Sky News that US defence secretary General James Mattis and national security adviser H R McMaster acted swiftly and "took advantage of those dreadful images of dead babies".
Sen McCain said the strikes were about both sending a message and the beginning of a campaign that could lead to the partition of Syria.
Fifty-nine Tomahawk missiles were fired from two warships - the USS Porter and USS Ross - in the Mediterranean Sea, targeting the government-controlled Shayrat airfield near the city of Homs.
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Mr Trump said the strikes targeted the airbase from which the chemical attack on a rebel-held town in the north of Syria was launched on Tuesday.
The missiles struck their targets, which also included Syrian aircraft and fuel stations, at 4.40am local time on Friday.
Syrian state TV described the US missile attack as "American aggression", and the state news agency said at least nine civilians, including four children, were killed.
Russia, however, said four Syrian soldiers were killed, two were missing and six wounded.
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The US action comes days after at least 80 people, many of them children, were killed in a poison gas attack in the town of Khan Sheikhoun, in Idlib province, that has been blamed on the Assad regime.
Shortly after the US campaign, airstrikes hit Khan Sheikhoun again, according to a witness and a war monitoring group.
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Downing Street said the UK Government "fully supports" the military action, calling it "an appropriate response to the barbaric chemical weapons attack" which "is intended to deter further attacks".
Mr Trump had said during the campaign that he would not be drawn into Syria's conflict. But the chemical attack changed his view on the situation, he said, as haunting images of harmed children shocked the world.
He called the attack "one of the truly egregious crimes" that "shouldn't have happened and it shouldn't be allowed to happen".
The Syrian government has denied being behind that attack and the Russian government warned against apportioning blame until an investigation had been carried out.
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US Secretary of State Rex Tillerson said the US strikes were "proportionate" and showed the President was willing to act when other countries "cross the line".
He also blamed Russia for failing to uphold a 2013 agreement to secure Syria's chemical weapons, saying Moscow had been either "complicit or incompetent".
Mr Trump's predecessor Barack Obama had threatened to attack Mr Assad's forces for previous chemical weapons attacks, but never went ahead.
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Mr Trump's intervention raises the risk of confrontation with Russia and Iran, Syria's two main military allies.
Russia has backed Syria since September 2015, also using its veto power in the UN Security Council on several occasions to prevent sanctions against Damascus.
The US informed Russian forces ahead of its attack on Syria and did not target sections of the base where they were believed to be present.