A defiant US President Donald Trump clashed with congressional Democrats Thursday before lawmakers hold a largely symbolic but politically charged vote to rein in his ability to take military action against Iran.

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.