US Secretary of State Mike Pompeo has acknowledged that the United States did not know "precisely" when or where the imminent attacks allegedly being planned by top Iranian commander Qassemi Soleimani would take place, but said the threat was "real".

"There is no doubt that there were a series of imminent attacks that were being plotted by Qassem Soleimani," Pompeo said in a Fox News interview that aired on Thursday.

"We don't know precisely when, and we don't know precisely where, but it was real," he added.

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Soleimani was assassinated in a US drone strike near Baghdad on January 3, prompting Iran to retaliate with a series of missile attacks on US facilities in Iraq several days later.

In its justification for the strike, the US has repeatedly said Soleimani was planning "imminent attacks" against US forces in the region. Officials have not publicly elaborated on the evidence of any planned attacks.

Two Republicans slam justification

Pompeo's recent comments came in response to a question about the criticism the Trump administration has received from members of Congress, including two Republican senators, over the recent escalation of tensions with Iran.

The House of Representatives and the Senate received separate briefings on Wednesday about the administration's decision to kill Soleimani.

Most Republicans defended Donald Trump, saying the US president made the "right call".

But two Republican senators - Mike Lee and Rand Paul - joined Democrats in slamming the briefings, calling them "insulting" and "demeaning".

"It's un-American. It's unconstitutional. And it's wrong," Lee told reporters after Wednesday's Senate briefing.

"I find it insulting and I find it demeaning to the Constitution of the United States," he said, adding the briefing was "probably the worst briefing I've seen at least on a military issue in the nine years I've served in the US Senate."

Senators Mike Lee and Rand Paul depart via the Senate Subway following a classified national security briefing of the US Senate on developments with Iran [Tom Brenner/Reuters]

The Democratic-controlled House on Thursday passed a nonbinding resolution aimed at limiting the president's ability to attack Iran in the future without congressional approval.

The House's War Powers resolution directs Trump to terminate military operations against Iran except for self-defence and clarifies that the president currently does not have congressional authority to engage in war with Iran. A similar version is expected to be debated in the Republican-controlled Senate, where it faces an uphill battle.

Under the US Constitution, the authority to direct military action is divided between Congress and the president. Congress has the power to declare war while the president, as commander-in-chief, has the power to use the military to defend the US.