US President Donald Trump has ordered a military attack against Syrian President Bashar al-Assad, joining allies Britain and France in launching missile strikes in retaliation for what Western nations said was the deliberate gassing of Syrian civilians.

The coordinated strike on Friday marked the second time in a year that Trump has used force against Assad, who US officials believe has continued to test the West's willingness to accept gruesome chemical attacks, The Washington Post reported.

Trump announced the strikes in an address to the nation on Friday evening.

"The purpose of our action tonight is to establish a strong deterrent," he said, against the production and use of chemical weapons, describing the issue as vital to national security.

Trump added that the US is prepared "to sustain this response" until its aims are met.

Trump asked both Russia and Iran, backers of Assad, "what kind of nation wants to be associated" with mass murder and suggested that someday the US might be able to "get along" with both if they change their policies.

US State Department spokesperson Heather Nauert had earlier on Friday said the US has "a very high level of confidence" that Syria was responsible for the recent chemical attack in the Middle East nation.

Nauert said in a press briefing on Friday that the US has proof that the Syrian government was behind this attack, and will "continue to gather information and further assess it", Xinhua news agency reported.

"I can tell you this. Syria's responsible. We are all in agreement," she said. "We can say that the Syrian government was behind this attack."

However, she refused to show the evidence, saying the thing was "sensitive."

She also refuted the accusation of Russia in the UN meeting earlier on Friday that Britain was behind the Syria attack, saying Moscow tried to "completely change the story" and "turn it upside down."

"The UK, I'm confident in saying, had absolutely nothing to do with it," she added. "It is the assessment of the US government, the British government, the French government."

Meanwhile, Russia's Defence Ministry Spokesman Lgor Konashenkov said that "we have evidence that proves Britain was directly involved in organising this provocation" in Douma.

Activists and rebels in Syria claimed that Syrian forces used chlorine gas in the attack on April 7 against Douma in the rebel-held area near Damascus.

The Syrian government has denied the allegations and called the reports fake news that the West uses to justify attacking the country.