Updated Friday April 7

US President Donald Trump said yesterday he ordered missile strikes against a Syrian airfield from which a deadly chemical weapons attack was launched, declaring he acted in America's "vital national security interest".

In a sharp escalation of the US military role in Syria, two US warships fired dozens of cruise missiles from the eastern Mediterranean Sea at the airbase controlled by President Bashar al-Assad's forces in response to the poison gas attack in a rebel-held area on Tuesday, US officials said.

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.

READ: Britain says it 'fully supports' the US strike

US officials said they informed Russian forces ahead of the missile attacks and that 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 from his Florida resort, Mar-a-Lago, where he was attending a summit with Chinese President Xi Jinping.

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.

Fifty-nine Tomahawk missiles were launched from the USS Porter and USS Ross around 8.40pm EDT (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 strikes occurred as Trump and Xi were wrapping up a dinner of Dover sole and dry-aged New York strip steak.

"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.

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

The attacks spurred a modest flight to safety in global financial markets, sending yields on safe-haven US Treasury securities to their lowest since November. Stocks weakened in Asia and US 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.

'PROPORTIONATE'

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

The governor of Syria's Homs province, where the airbase is located, said the strikes were not believed to have caused "big human casualties" but had caused material damage.

Governor Talal Barazi told Reuters the attack served the interests of "armed terrorist groups" including Islamic State, adding that the targeted air base had been providing air support for army operations against Islamic State east of Palmyra.

"I believe - God willing - that the human casualties are not big, but there is material damage. We hope there are not many victims and martyrs," he said by telephone.

Trump said: "Syrian dictator Bashar al-Assad launched a horrible chemical weapons attack on innocent civilians ... 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," he said.

Trump appeared to have opted for measured and targeted air attacks instead of a full-blown assault on Assad's forces and installations.

"We feel the strike itself was proportionate," said US Secretary of State Rex Tillerson.

According to a US defence official, Trump first asked about possible military action on Wednesday, after US intelligence agencies confirmed that Syrian aircraft based at the al Shayrat airbase had dropped Sarin gas on civilians.

Planning began on Wednesday and accelerated at the Pentagon, the State Department and the White House on Thursday, helped by the fact that the Defense Department had numerous off-the-shelf plans, including for cruise missile strikes on Syrian airfields.

"It was a matter of dusting those off and adapting them for the current target set and timing," the official told Reuters, speaking on condition of anonymity.

AT ODDS WITH RUSSIA

The relatively quick response to the chemical attack came as Trump faced a growing list of global problems, from North Korea and China to Iran and Islamic State, and may have been intended to send a message to friends and foes alike of his resolve to use military force if deemed necessary.

"One question is whether Russia will respond in any meaningful way," said a senior US official involved in planning the raid. "If they do, they will be further complicit in the actions of the Syrian regime."

The attack has put Trump 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 Islamic State militants in northern Syria, where U.S. special forces are supporting Arab and Kurdish armed groups.

Speaking just before the strikes were announced, Russia's deputy U.N. envoy, Vladimir Safronkov, warned of "negative consequences" if the United States went ahead with military action, saying the blame would be "on shoulders of those who initiated such doubtful and tragic enterprise."

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.

US lawmakers had a mixed reaction, with some criticising 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 UN Security Council was expected to hold closed-door consultations on Friday about the US strike on Syria following a request by Bolivia, an elected member of the council, a senior Security Council diplomat said.