With the resolution triggering a scalding debate about presidential war powers amid heightened tensions with Iran, Trump insisted he does not need lawmakers' blessings to launch attacks, essentially scorning existing legal requirements for consulting with Congress.

"I don't have to," Trump said when asked whether he would seek congressional approval for more military action against Iran, as a war powers debate raged in the House of Representatives.

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

Trump signaled 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 housing American forces in Iraq.

But Democrats, and two Senate Republicans, have expressed deep skepticism about the administration's rationale for killing 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.

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

She also slammed the administration's "cavalier attitude" and apparent lack of a coherent strategy to ensure regional stability and Americans' safety.

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 stinging rebuke to Trump's foreign policy.

Trump tweeted that he was counting on all House Republicans to "vote against Crazy Nancy Pelosi's War Powers Resolution."

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."

- 'Un-American' -

Pelosi said her Democrats were moving 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.

After clearing the House the measure would face a steep climb in the Senate, where Republicans hold a 53-47 majority.

But two Senate Republicans, Mike Lee and Rand Paul, announced support for a war powers resolution, suggesting a potential razor-thin vote ahead.

The pair emerged from the top-secret briefing saying administration officials provided no acceptable rationale for killing Soleimani, nor any specific evidence of imminent threats against Americans, as Trump has claimed.

An outraged Lee said the briefers, who included Secretary of Defense Mark Esper and CIA Director Gina Haspel, were "insulting" by 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.

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

In the House, Republican congressman Andy Barr passionately defended what he said was Trump's inherent constitutional authority to act without Congress to defend the nation.

But congresswoman Ayanna Pressley pushed back, demanding forthrightness from the administration about the reasons for sending US troops into battle.

"We cannot allow our country to be lied into another war," she said.