US President Donald Trump announced the military action from the White House late on Friday. As he spoke, explosions rocked Damascus.

Trump said he was prepared to sustain the response until Assad's government stopped its use of chemical weapons.

British Prime Minister Theresa May and French President Emmanuel Macron said the UK and France had joined in the attack.

Syria's seven-year-old civil war has pitted the United States and its allies against Russia, which itself intervened in the war in 2015 to back Assad.

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Syrian state media said the attack would fail and called it a "flagrant violation of international law".

Russia's ambassador to the United States said Moscow had warned that "such actions will not be left without consequences".

In a televised address from the White House, Trump said he had ordered US armed forces to launch "precision strikes" associated with Assad's chemical weapons capabilities.

Speaking of Assad and his suspected role in last week's chemical weapons attack, Trump said: "These are not the actions of a man. They are crimes of a monster instead."

At least six loud explosions were heard in Damascus in the early hours of Saturday and smoke was seen rising over the Syrian capital, 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 center.

At a Pentagon briefing, Chairman of the Joint Chiefs of Staff General Joseph Dunford said the targets included a Syrian research facility and a chemical weapons storage facility.

The combined US, British and French assault appeared to be more intense than the strike Trump ordered almost exactly a year ago against a Syrian airbase in retaliation for an earlier chemical weapons attack attributed to Assad.

Defense Secretary Jim Mattis said twice as many weapons were used in the strikes on Saturday compared to last April's strike.

Mattis described the strikes as a "one time shot" to dissuade Assad from "doing this again".

Dunford said that manned US aircraft were used in the military operation and that the strike was planned to minimise the risk of casualties among Russia's military forces in Syria.

A US official told Reuters that Tomahawk cruise missiles also were involved.

Syrian air defences shot down 13 missiles, Syrian state TV said. The missiles had been shot down in the Kiswah area south of the capital Damascus, it said.

The Syrian Observatory for Human Rights said a total of three scientific research centres had been hit in the attacks, two in Damascus and one in the Homs area, in addition to military bases in Damascus.

"The purpose of our actions tonight is to establish a strong deterrent against the production, spread and use of chemical weapons," Trump said in his televised address.

The US president, who has tried to build good relations with Russian President Vladimir Putin, was critical of Russia and Iran, which have backed Assad's government.

"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.

British Prime Minister Theresa May said she had authorised British armed forces "to conduct coordinated and targeted strikes to degrade the Syrian regime's chemical weapons capability".

"This is not about intervening in a civil war. It is not about regime change," May said in a statement.

"It is about a limited and targeted strike that does not further escalate tensions in the region and that does everything possible to prevent civilian casualties."

Trump made clear in his eight-minute televised address that he was wary of a deeper entanglement in Syria, where about 2000 US troops have been deployed to fight Islamic State.

"If it is finished, and there is no second round, it will be considered limited," the official told Reuters.

The air strikes, however, risk dragging the United States further into Syria's civil war, particularly if Russia, Iran and Assad opt to retaliate.

Australian Associated Press