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

Just hours after US 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 US 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, US officials said.

Putin, a staunch ally of Assad, regarded the US 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 most direct US action yet in Syria's six-year-old civil war and risks leaving Trump facing his biggest foreign policy crisis since his inauguration on 20 January, raising the risk of confrontation with Russia and Iran, Assad's two main military backers.

US officials said that 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.

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

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.

Iran also condemned the move, while Britain and Turkey gave their support, with Australian Prime Minister Malcolm Turnbull calling it a 'proportionate' response to the use of chemical weapons.

Iran denounced the 'destructive and dangerous' strike, the Students News Agency ISNA quoted a Foreign Ministry spokesman as saying.

'Iran strongly condemns any such unilateral strikes ... Such measures will strengthen terrorists in Syria ... and will complicate the situation in Syria and the region,' ISNA quoted Bahram Qasemi as saying.

Britain gave its backing. 'The UK Government fully supports the US action, which we believe was an appropriate response to the barbaric chemical weapons attack launched by the Syrian regime and is intended to deter further attacks,' a government spokesman said.

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 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,' Trump said later. 'Tonight I ordered a targeted military strike on the airfield in Syria from where the chemical attack was launched.'

Trump added: 'It is in this vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons.'

Additional reporting by Reuters.