US President Donald Trump braced for a brawl with congressional Democrats who were set to vote on Thursday to rein in his ability to take military action against Iran.

The largely symbolic resolution was sure to trigger a scalding debate about presidential war powers at a time of heightened tensions with Tehran, a long-time US adversary.

Trump signalled on Wednesday he was stepping back from the brink of war with the Islamic republic after a US drone strike that killed its top commander was followed by Iranian missile volleys against bases that house US forces in Iraq.

But Democrats, and two Senate Republicans, have expressed deep scepticism about the administration's rationale for Trump's order to kill general Qasem Soleimani, and are demanding Congress reassert its power over a commander in chief's use of American military might against another nation.

Citing the War Powers Resolution of 1973 which forbids a 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, introduced by congresswoman Elissa Slotkin, a former CIA officer with extensive experience in Iraq, also provides for key exceptions, allowing use of force to defend against or prevent an “imminent” attack against the United States or Americans.

The measure has been introduced in the House 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 stinging rebuke to Trump's foreign policy strategy.

Trump said on Thursday ahead of the vote that he was counting on his Republican Party to present a united front against the measure.

“Hope that all House Republicans will vote against Crazy Nancy Pelosi's War Powers Resolution,” the president tweeted.

Pelosi, the speaker of the House, said her Democrats would move forward because their concerns were not addressed in a closed-door briefing to lawmakers Wednesday by Secretary of State Mike Pompeo and other top officials.

“The president has made clear that he does not have a coherent strategy to keep the American people safe, achieve de-escalation with Iran and ensure stability in the region,” Pelosi said.

'Un-American'

Should the measure clear the House as expected, it would face a steep climb in the Senate, where Republicans hold a 53-47 majority.

But two Senate Republicans, Mike Lee and Rand Paul, came out in favor of a war powers resolution, suggesting a potential razor-thin vote.

They emerged from the top-secret briefing saying administration officials provided no acceptable rationale for the killing of Soleimani, or any specific evidence of an imminent threat against US forces or citizens.

An outraged Lee said the briefers, who included Secretary of Defence Mark Esper and CIA Director Gina Haspel, were “insulting” by discouraging questions about military policy towards Iran.

“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.

The stunning criticism marked an extraordinary moment on Capitol Hill, given the normally unified Republican support for the US national security apparatus.

Senator Lindsey Graham, a Trump loyalist and defence hawk, inserted himself in the debate by warning Lee and Paul that they were “empowering the enemy” by supporting a resolution to curtail presidential warmaking powers.

Paul clapped back on CNN, accusing Graham of invoking a false “drape of patriotism” and failing to understand the US Constitution and its separation of powers.