US defence secretary James Mattis reportedly urged Donald Trump to seek congressional approval for airstrikes on Syria but was overruled by the president, who wanted to be seen as backing up his bellicose tweets with action.

Mr Mattis warned an overly aggressive response to an alleged chemical attack in Douma risked igniting wider conflict with Russia and stressed the importance of public support for military campaigns, government officials told the New York Times.

However, Mr Trump is said to have been keen to act upon his combative Twitter posts, in which he told Russia to “get ready” because missiles “will be coming” to Syria and warned Moscow it “shouldn’t be partners with a Gas Killing Animal who kills his people and enjoys it”.

He gave the go-ahead to Saturday morning’s multi-target “precision strikes”, backed by the UK and France, without consulting Congress.

Mr Trump said the strikes were “in the vital national security and foreign policy interests” of the US, but operation was branded “illegal” by Democrats and criticised by some Republican members of Congress.

While Mr Mattis lost the debate over seeking political approval, however, he successfully made the case for limiting the scale of the military action, according to Pentagon and White House officials who the New York Times did not name.

On Thursday, White House Press Secretary Sarah Sanders, denied the reports – saying in a statement: “Reports that Secretary Mattis urged the President to seek congressional approval before last week’s strikes in Syria are categorically false.  As Secretary Mattis explained to Congress in yesterday’s all-member briefs, the President appropriately ordered the strikes under his constitutional authorities.”

The US fired missiles at three sites linked to chemical weapons and carefully avoided Russian or Iranian causalities.

They did not target Syrian president Bashar al-Assad, and Mr Mattis was quick to stress the “one-off” operation was aimed only at military installations rather than the regime’s political leadership.

Mr Trump had previously suggested the airstrikes could begin a drawn-out campaign on the Syrian government.

“We are prepared to sustain this response until the Syrian regime stops its use of prohibited chemical agents,” he said.

“The strike was really just enough to cover the president politically, but not enough to spark a war with the Russians,” said Jon Soltz, a former US army soldier who served in the Iraq war and now chairs the liberal veterans group VoteVets.

He added: “It was clear the military had tight constraints on the operation, and that everybody in the military seemed to know that except the president.”

Mr Mattis and Joint Chiefs of Staff chairman Joseph Dunford briefed the Senate and House of Representatives about Syria behind closed doors on Tuesday afternoon, congressional aides said.

Lawmakers had been pushing Mr Trump’s administration for more information about its Syria policy since the weekend’s bombing.