The United States on Friday fired dozens of cruise missiles at a Syrian airbase from which it said a deadly chemical weapons attack was launched this week, an escalation of the U.S. military role in Syria that immediately raised tension with Russia.

Just hours after U.S. President Donald Trump announced he had ordered the attack, a spokesman for Russian President Vladimir Putin said the strike had seriously damaged ties between Washington and Moscow.

Two U.S. warships fired 59 cruise missiles from the eastern Mediterranean Sea at the Syrian airbase controlled by the forces of President Bashar al-Assad in response to a poison gas attack in a rebel-held area on Tuesday, U.S. officials said.

Putin, a staunch ally of Assad, regarded the U.S. action as "aggression against a sovereign nation" on a "made-up pretext" and a cynical attempt to distract the world from civilian deaths in Iraq, his spokesman, Dmitry Peskov, was cited as saying by agencies.

It was the toughest direct U.S. action yet in Syria's six-year-old civil war and leaves Trump facing his biggest foreign policy crisis since his Jan. 20 inauguration, raising the risk of confrontation with Russia and Iran, Assad's two main military backers.

U.S. officials said they informed Russian forces ahead of the missile attacks and that they took pains to avoid hitting Russian troops at the base, saying there were no strikes on sections of the base where Russians were present. But they said the administration did not seek Moscow's approval.

"Years of previous attempts at changing Assad’s behaviour have all failed and failed very dramatically," Trump said as he announced the attack from his Florida resort, Mar-a-Lago, where he was meeting Chinese President Xi Jinping.

Trump ordered the strikes a day after he blamed 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.

The Tomahawk missiles were launched from the USS Porter and USS Ross around 0040 GMT on Friday, striking multiple targets - including the airstrip, aircraft and fuel stations - on the Shayrat Air Base, which the Pentagon says was used to store chemical weapons.

The attack was a "one-off," a U.S. defence official told Reuters, meaning it was expected to be a single strike with no current plans for escalation.

U.S. Secretary of State Rex Tillerson said the strike did not mean the wider U.S. policy on Syria had changed.

"This clearly indicates the president is willing to take decisive action when called for," he told reporters. "I would not in any way attempt to extrapolate that to a change in our policy or our posture relative to our military activities in Syria today. There has been no change in that status."

The attacks spurred a flight to safety in global financial markets, sending yields on safe-haven U.S. Treasury securities to their lowest since November. Stocks weakened in Asia and U.S. equity index futures slid, indicating Wall Street would open lower on Friday. Prices for oil and gold both rose, and the dollar slipped against the Japanese yen.

ARMY SAYS SIX KILLED

The Syrian army said six people were killed in the attack which led to big material losses. The Syrian Observatory for Human Rights said at least four Syrian soldiers, including a senior officer, were killed in the strikes, which almost completely destroyed the base.

"Initial indications are that this strike has severely damaged or destroyed Syrian aircraft and support infrastructure and equipment at Shayrat Airfield, reducing the Syrian government's ability to deliver chemical weapons," said Pentagon spokesman Captain Jeff Davis.

Syrian state TV said that "American aggression" had targeted the base with "a number of missiles" and cited a Syrian military source as saying the strike had "led to losses".

Trump sought to cast the attack, which took place as he and Xi were wrapping up a dinner of Dover sole and dry-aged New York strip steak, as an effort to deter Syria from using chemical weapons in the future.

"Syrian dictator Bashar al-Assad launched a horrible chemical weapons attack on innocent civilians," he said later. "Tonight I ordered a targeted military strike on the airfield in Syria from where 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," Trump added.

Iran, which also backs Assad, denounced the attack.

"Iran ... condemns use of chemical weapons ... but at the same time believes it is dangerous, destructive and violation of international laws to use it as an excuse to take unilateral actions," Students News Agency ISNA quoted Foreign Ministry spokesman Bahram Qasemi as saying.

Israel welcomed the move.

"In both word and action, President Trump sent a strong and clear message today that the use and spread of chemical weapons will not be tolerated," Prime Minister Benjamin Netanyahu's office said in a statement.

U.S. lawmakers had a mixed reaction, with some criticizing Trump's decision to use force without getting their approval.

"Congress will work with the president, but his failure to seek congressional approval is unlawful,” said Senator Tim Kaine, the 2016 Democratic vice presidential candidate.

The U.N. Security Council was expected to hold closed-door consultations on Friday about the U.S. strike on Syria following a request by Bolivia, an elected member of the council, a senior Security Council diplomat said.