But House Speaker Nancy Pelosi said she wanted the entire Congress to be briefed by next week ahead of a week-long recess so that all lawmakers can be brought up to speed on any new intelligence developments before debating any next steps.

"We're hoping that for sure that before the break we will have a classified briefing on the Middle East, on Iran before the full House of Representatives," Pelosi told reporters.

It was not immediately clear who would conduct the briefing later Thursday, expected to include Pelosi, House Minority Leader Kevin McCarthy, Republican Senate Majority Leader Mitch McConnell, top Senate Democrat Chuck Schumer, and the four top leaders of the House and Senate intelligence committees.

The United States on Wednesday ordered non-emergency staff evacuated from its Baghdad embassy due to an "imminent" threat from Iranian-linked Iraqi militias, a move that prompted concern on Capitol Hill.

Strains have ratcheted up with the US deploying an aircraft carrier group and B-52 bombers to the Gulf, and Iran denouncing what it sees as "unacceptable" provocations by Washington.

World powers have rushed to urge calm and US allies continued to show skepticism over Washington's alarm bells.

Opponents of President Donald Trump say hardliners led by national security advisor John Bolton, who has long advocated toppling the Iranian regime, are pushing the country into war.

Pelosi issued her own warning, saying Congress, not the president, has the constitutional power to declare war, and she was quick to note Trump's repeated opposition to the US military intervention in Iraq.

"I like what I hear from the president, that he has no appetite for this," she said.

"One of the places that I agree with the president is in both of our opposition to the war in Iraq, and I hope that that same attitude will prevail with (Trump), even though some of his supporters are rattling sabers."