Democratic — and a few Republican — lawmakers were infuriated by a classified briefing they received from the Trump administration concerning the US military's strike on the Iranian leader Qassem Soleimani.

Democratic Sen. Chris Murphy of Connecticut said lawmakers weren't provided evidence of the "specific, imminent threat" the administration says it had before assassinating one of Iran's most powerful officials.

Trump must show he acted to defend US forces or the nation from an "imminent" threat to be in compliance with US and international law.

Notably, Republican Sen. Mike Lee of Utah lashed out at the administration following the briefing, calling it "probably the worst briefing, at least on a military issue, I've seen in nine years I've been here."

The Trump administration insists it was forced to "act in self-defense" and kill Soleimani to prevent an attack on the US within "days," but it has provided no information about the specific threat.

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Democratic — and a few Republican — lawmakers were infuriated by a classified briefing they received from the Trump administration on Wednesday concerning the US military's deadly strike on the Iranian military leader Qassem Soleimani.

Democratic Sen. Chris Murphy of Connecticut said lawmakers weren't provided evidence of the "specific, imminent threat" the administration says it had before assassinating one of Iran's most powerful officials.

He said the administration should have sought authorization from Congress to attack Iran, arguing the action wasn't covered by the 2001 Authorization for Use of Military Force.

"I did not receive any information in this briefing about a specific, imminent threat to US forces in the region," Murphy said. "I do not have confidence that this attack on Qassem Soleimani was warranted without congressional authorization."

Defense Secretary Mark Esper insisted on Tuesday that the government was forced to "act in self-defense" and kill Soleimani last week to prevent an attack on the US within "days" but has not made public any information regarding a specific threat.

Trump must show he acted to defend US forces or the nation from an "imminent" threat to be in compliance with US and international law.

Notably, Republican Sen. Mike Lee of Utah lashed out at the administration following the briefing, calling it "probably the worst briefing, at least on a military issue, I've seen in nine years I've been here."

Lee said the administration's refusal to seek authorization from Congress to kill a top Iranian government official, and thus dramatically escalate conflict with Iran, was "insulting," "demeaning," and "unconstitutional."

"It's un-American, it's unconstitutional, and it's wrong," he added.

Lee told reporters that he "walked into that briefing undecided" on whether to support a resolution pushed by Democratic Sen. Tim Kaine of Virginia that would limit the Trump administration's ability to take further military action against Iran without congressional authorization.

"That briefing is what changed my mind," Lee said. "I'm now going to support it."

He added, "Drive-by notification or after-the-fact lame briefings like the one we just received aren't adequate."

Murphy, who called Trump's escalation of conflict with Iran "a disaster of epic proportions to US national security interests," added that the Trump administration allowed just one hour and 15 minutes for the briefing and that only about 15 senators were able to ask questions.

"It also appeared to me that just as the questions were getting tougher about whether or not there was a specific, imminent threat, the administration decided to leave the room," he said.

Senate Minority Leader Chuck Schumer said he asked the administration to send the officials back to Congress in a week to continue briefing lawmakers but hadn't received a commitment.

Democratic Rep. Pramila Jayapal, a cochair of the House Progressive Caucus, also said the administration provided "no evidence of an imminent threat or attack."

And Rep. Tulsi Gabbard, a 2020 Democratic presidential candidate, said the administration provided "no justification whatsoever for this illegal and unconstitutional act of war that President Trump took."

Trump's national security adviser, Robert O'Brien, said Soleimani was plotting "to attack American facilities and diplomats" at sites occupied by US "soldiers, sailors, airmen, and Marines."

The Pentagon as well as Secretary of State Mike Pompeo have repeatedly said the US attacked Iran based on intelligence that pointed to "imminent threats to American lives."

The Trump administration has refused to disclose any information about the intelligence that led to the US's dramatic escalation, however, and the government did not use the term "imminent" to describe Iran's planned attacks in its original statement justifying the strike.

"General Soleimani was actively developing plans to attack American diplomats and service members in Iraq and throughout the region," the January 2 statement said. "This strike was aimed at deterring future Iranian attack plans."

Sonam Sheth contributed to this report.