WASHINGTON－US lawmakers concerned about rushing to war with Iran adopted a measure on Thursday aimed at reining in US President Donald Trump's ability to take military action against the Islamic republic.

The resolution was introduced by Democrats in the House of Representatives after Trump's order to kill an Iranian commander and retaliatory missile strikes by Teheran dramatically escalated tensions and raised fears of a devastating war between the two foes.

The mostly symbolic but politically charged vote, 224 to 194, was largely along party lines, with three members of Trump's Republican Party joining Democrats in approving the measure demanding the president not engage in military action against Iran unless authorized by US Congress.

Among them was Matt Gaetz, one of Trump's staunchest supporters in US Congress, who noted in a floor speech that the measure did not criticize Trump, but said that "engaging in another forever war in the Middle East would be the wrong decision".

"If the members of our armed services have the courage to go and fight and die in these wars, as Congress we ought to have the courage to vote for them or against them," Gaetz said.

As lawmakers launched a scalding daylong debate over presidential authority, Trump insisted he needs no one's blessing to launch attacks, essentially scorning existing legal requirements for consulting Congress.

"I don't have to," Trump said when asked whether he would seek congressional approval for more military action against Iran.

"And you shouldn't have to," he said, "because you have to make split-second decisions sometimes."

Trump signaled on Wednesday he was stepping back from the brink of war with Iran after the US drone strike that killed Iranian general Qasem Soleimani was followed by Iranian missile volleys against bases housing US forces in Iraq.

But on Thursday, he fought back against criticism that he'd ordered the killing, risking all-out conflict, without real justification.

At a reelection campaign rally in Toledo, Ohio, Trump insisted, without providing any evidence, that Soleimani was "actively planning new attacks", including against US embassies, "and we stopped him cold".

He ridiculed his Democratic opponents in Congress, calling them insulting names and claimed that if he had consulted with them they would have leaked the secret operation to the "fake news".

"You should get permission from Congress," he said mockingly to mimic the Democratic House Speaker, Nancy Pelosi.

'Cannot afford war'

But Democrats, and some Republicans, have expressed deep skepticism about the administration's rationale for Soleimani's killing, and are demanding Congress reassert its power over a commander-in-chief's use of US military might against another nation.

Citing the War Powers Resolution of 1973 that forbids a US president from taking the country to war without congressional approval, the measure "directs the president to terminate the use of United States armed forces to engage in hostilities in or against Iran or any part of its government or military".

But the text also provides for key exceptions, allowing use of force to defend against or prevent an "imminent" attack against US citizens.

Pelosi, speaking to reporters about the resolution, said Trump "must de-escalate and must prevent further violence. America and the world cannot afford war".

The House measure was introduced as a concurrent resolution, a form of legislation that does not carry the weight of law. But as a political instrument it could serve as a rebuke to Trump's foreign policy.

Top House Republican Kevin McCarthy blasted Pelosi's effort as a "show vote" because it cannot become law and will therefore "never limit (Trump's) constitutional authority to defend the American people".

Pelosi said her Democrats were moving forward because their concerns were not addressed in a closed-door briefing to lawmakers on Wednesday by US Secretary of State Mike Pompeo and other top officials.

Democrats have introduced a similar war powers resolution in the Senate, where it faces a steep climb as Republicans hold a 53-47 majority.

Two Senate Republicans, Mike Lee and Rand Paul, announced support for the Senate version, suggesting the vote, as early as next week, could be razor-thin.

The pair emerged from the top-secret briefing saying administration officials provided no acceptable rationale for killing Soleimani or specific evidence of imminent threats against US citizens, and took the "insulting" step of discouraging questions about military policy.

"To come in and tell us that we can't debate and discuss the appropriateness of military intervention against Iran? It's un-American, it's unconstitutional, and it's wrong," Lee told reporters.