Trump administration officials failed to convince many US lawmakers on Wednesday that an imminent threat had justified the killing of a top Iranian military commander, and congressional Democrats scheduled a vote on legislation to rein in the president's ability to wage war.

Secretary of State Mike Pompeo, Secretary of Defence Mark Esper, Joint Chiefs of Staff Chairman Mark Milley and CIA Director Gina Haspel held classified briefings for all 535 members of Congress to discuss President Donald Trump's decision to order a drone strike that killed elite Quds Force commander Qasem Soleimani in Iraq last week.

The House will vote Thursday on a measure limiting Trump's ability to take military action against Iran as Democratic criticism of the US killing of the top Iranian general intensified.

Following the sessions, Democrats and a few Republicans said the officials had not provided evidence to back up assertions by Trump and military commanders that Soleimani had posed an "imminent threat" to the United States, and they disputed the administration's argument that the killing of a foreign leader in a third country was legally justified.

"The basic theme of it was the administration essentially saying: Trust us. And that's really what it all boils down to. I'm not sure who I trust or what I trust when it comes to these issues because we've been told so many different things that really just bother me," Representative Eliot Engel, chairman of the House Foreign Affairs Committee, told reporters.

US House of Representatives Speaker Nancy Pelosi said shortly afterward that the Democratic-controlled chamber would vote on a war powers resolution as soon as Thursday.

"Members of Congress have serious, urgent concerns about the Administration's decision to engage in hostilities against Iran and about its lack of strategy moving forward," Pelosi said.

She said Trump's report to Congress about the strike and the briefings had not addressed members' concerns.

The war powers resolution directs Trump to terminate the use of US Armed Forces in or against Iran, unless Congress has declared war or passed an authorization for the use of force against it.

Retaliation

The classified briefings came hours after Iranian forces fired missiles at military bases housing US troops in Iraq, in retaliation for the killing of the Iranian general, raising the stakes in its conflict with Washington amid concern of a wider war in the Middle East.

Earlier on Wednesday, Trump tempered days of angry rhetoric in a nationally televised address to suggest that Iran was "standing down" after firing its ballistic missiles, raising hopes that both Washington and Tehran were defusing the crisis.

The resolution is expected to easily pass the House, but would face a more difficult time in the Senate, which is controlled by Trump's fellow Republicans.

Senator Jim Risch, the Republican chairman of the Senate Foreign Relations Committee, said it was among the best briefings he had ever attended.

Senator Roy Blunt, a member of the Senate Intelligence Committee, said Soleimani was the "No. 1 terrorist mastermind" and that he thought Trump had sent a strong and important signal to America's adversaries and its friends "that the president is going to make the hard decisions".

But Republican Senator Mike Lee said the briefing was the worst he had heard in nine years in the Senate, at least on a military issue and that he would vote for the war powers resolution.

"What I found so distressing about that briefing was that one of the messages we received from the briefers was do not debate, do not discuss the issue of the appropriateness of further military intervention against Iran and that if you do, you will be emboldening Iran," Lee said.

Trump broke precedent by failing to inform congressional leaders before ordering the drone strike. He angered some lawmakers, particularly Democrats, by making classified his formal report to Congress about the strike as he sent more troops to the Middle East.

The US constitution grants Congress, not the president, the right to declare war. Some lawmakers have been trying for years — under Democratic President Barack Obama as well as under Trump — to wrest back that authority from the White House.