US lawmakers were left insulted and dumbfounded by the Trump administration's briefing to explain why the US targeted and killed Iranian Major General Qassem Soleimani.

"I find this insulting and demeaning," said Senator Mike Lee from Utah, a Republican.

"No case was made for imminence.  No case was made for thinking this through, and I leave more troubled than when I went into it," said Rep. Jerry Connolly, a Virginia Democrat who added the briefing led by Secretary of State Mike Pompeo was "sophomoric and unconvincing".

The briefing was classified, but the reactions from indignant lawmakers were not.  Many from both major parties indicated that Donald Trump may not have had a pressing reason to order last week's air strike that resulted in the assassination of Soleimani.  Trump and several of his officials had maintained that the leader of the Iranian military's elite Quds force was planning imminent attacks on Americans and American interests.  A week after the air strike, they've yet to produce any evidence to back their claim.

"It was probably the worst briefing I've seen at least on a military issue in the nine years I've served in the United States Senate," Senator Lee continued.  He said he left the briefing "somewhat unsatisfied" with the White House's "legal, factual and moral justification for the attack."

Standing next to his fellow Republican, Kentucky Senator Rand Paul concurred, adding, "Today, this is Senator Lee and I saying, we are not abdicating our duty.

Lee says he is now planning to vote for a war powers resolution.  When Thursday rolls around to Washington, the House will vote on the resolution that would rein in Trump's ability to take military action against Iran without congressional approval.  Democrat Tim Kaine will introduce it to the Senate.