Moscow slammed the United States strike on a Syrian airbase on Friday as "aggression against a sovereign state", and suspended a bilateral agreement to help avoid clashes in the skies over the war-torn country.
"President Putin considers American strikes on Syria aggression against a sovereign state in violation of international norms, and under an invented pretext," said the statement by the Kremlin press service posted on the official website.
US President Donald Trump ordered the military strike on the Syrian air base at 0040GMT, in retaliation for what he said was a "very barbaric attack" Tuesday that is suspected to have contained a nerve agent.
The Syrian army has said the missile attack killed six people and caused extensive damage. It described the attack as an act of "blatant aggression", saying it had made the US "a partner" of Islamic State, the Nusra Front and other "terrorist organisations".
Watch: Syrian army spokesman on US attack
Russia's foreign ministry announced on Friday the suspension of a memorandum signed with the US in October 2015 which set up a hotline to avoid clashes between their air forces in Syrian airspace as they carried out separate bombing campaigns.
"We call upon the UN Security Council to hold an extraordinary meeting to discuss the situation," the ministry said in a statement, calling the strike "thoughtless."
Russia had sought to deflect blame from Assad over the incident, claiming Syrian jets struck a rebel arms depot housing "toxic substances" and denying that the regime has access to any chemical weapons.
Moscow, which launched a military intervention in support of Assad's forces in 2015, said the missile strike would "inflict major damage on US-Russia ties."
US officials and allies described the attack, which saw 59 tomahawk missiles launched from US Navy warships in the Mediterranean Sea, as a one-off that would not lead to further escalation.
A Pentagon spokesman said the missiles targeted "aircraft, hardened aircraft shelters, petroleum and logistical storage areas, ammunition supply bunkers, air defence systems and radars".
The spokesman said Russian forces were believed to be present on the base and those sections were not targeted.
The attack was hailed by the Syrian opposition and supported by US allies including Australia, Britain, France, Germany, Israel, Japan and Saudi Arabia.
NATO ally Turkey, which is a key player in the Syria conflict and has endured choppy relations with Washington recently, welcomed the strikes as "positive".
Beijing offered a nuanced reaction, saying it was "urgent" to avoid "further deterioration of the situation."
A foreign ministry spokeswoman added: "We oppose use of chemical weapons by any country, organisation or individual in any circumstance, for any purpose."
The governor of the Syrian province of Homs, Talal Barazi, said five people were killed and seven wounded on the airbase and there were also casualties in the village nearby.
The Syrian Observatory for Human Rights, a Britain-based monitor, said seven Syrian military personnel had been killed in the strike.
"The airbase was almost completely destroyed - the runway, the fuel tanks and the air defences were all blown to pieces," the Britain-based monitoring group said.
In a three minute statement issued about 9.40pm in the US, Trump confirmed he had ordered the attack on the airfield from which the chemical attack was launched.
"It is in this vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons," he said.
"There can be no dispute that Syria used banned chemical weapons, violated its obligations under the chemical weapons convention and ignored the urging of the UN Security Council.
Watch: US President Donald Trump launches strike action against Syria
The US military said initial indications were that the strike had severely damaged or destroyed Syrian aircraft, support infrastructure and equipment at the airbase.
This would reduce the Syrian government's ability to deliver chemical weapons, the statement said.
The Syrian National Coalition opposition group welcomed the missile strikes, a spokesperson said.
The spokesperson said the SNC hoped the US would continue to order strikes to stop the Syrian government's attack and "use of internationally banned weapons".
Syrian state TV confirmed "American aggression" had targeted the based with "a number of missiles" and that it had "led to losses", but gave no further details.
'End the slaughter and bloodshed'
Trump called on "all civilised nations to join us in seeking to end the slaughter and bloodshed in Syria".
"We hope that as long as America stands for justice, then peace and harmony will, in the end, prevail," he said.
"It was a slow and brutal death for so many men, women and children. Even beautiful babies were cruelly murdered in this barbaric attack. No child of God should ever suffer such horror.
"The refugee crisis continues to deepen and the region continues to destabilise threatening the US and its allies.
"To end terrorism of all kinds and all types. We ask for God's wisdom as we face the challenge of our very troubled world."
Facing his biggest foreign policy crisis since taking office in January, Trump took the toughest direct US action yet in Syria's six-year-old civil war, raising the risk of confrontation with Russia and Iran, Assad's two main
military backers.
US Secretary of State Rex Tillerson accused Russia of failing to carry out a 2013 agreement to secure Syrian chemical weapons.
He said Moscow was either complicit or incompetent in its ability to carry out the agreement.
"The strikes show Trump is prepared the take decisive action to respond to heinous acts," Tillerson said.
Tillerson said the US had a "high degree of confidence" that sarin nerve gas was used in the chemical attack on civilians.
The Pentagon said the US informed Russian forces ahead of the missile strike on the airfield, but Tillerson said "we sought no approval from Moscow".
Watch: Trump calls for support against Syria
Chemical weapons attack response
Trump ordered the strikes just a day after he pointed the finger at Assad for this week's chemical attack, which killed at least 70 people, many of them children, in the Syrian town of Khan Sheikhoun. The Syrian government has denied it was behind the attack.
Trump, who was attending a summit with Chinese President Xi Jinping at his Florida resort, said earlier on Thursday that "something should happen" with Assad as the White House and Pentagon studied military options.
US military action put the new president at odds with Russia, which has air and ground forces in Syria after intervening there on Assad's side in 2015 and turning the tide against mostly Sunni Muslim rebel groups.
Trump has until now focused his Syria policy almost exclusively on defeating IS militants in northern Syria, where US special forces are supporting Arab and Kurdish armed groups.
The risks have grown worse since 2013, when Barack Obama, Trump's predecessor, considered and then rejected ordering a cruise missile strike in response to the use of chemical weapons by Assad's loyalists.
It’s the first direct American assault on the Syrian government and Donald Trump's most dramatic military order since becoming president.
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