US President Donald Trump authorised the killing of top Iranian military commander Qassem Soleimani seven months ago, NBC News reported, raising new questions about the justification the administration has given for ordering the assassination.

Citing five current and former senior administration officials, NBC News reported that the directive issued in June 2019 came with the conditions that Trump would have final sign-off on any operation to kill Soleimani and the option would only be used if an American was killed by Iran.

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The option was initially discussed following Iran's downing of an unmanned US military drone over the Gulf, NBC reported. Iran maintained the drone was shot down over its waters, but the Trump administration said it was flying in international airspace at the time.

Then-White House National Security Adviser John Bolton had urged Trump to retaliate for the drone downing by signing off on an operation to kill Soleimani, NBC reported, citing officials briefed on the discussions at the time. Secretary of State Mike Pompeo also supported the option, but Trump said he would only take the step if Iran killed an American, the US news outlet said.

The White House did not immediately comment on the report.

The report raises new questions about the Trump administration's given justification for its decision to order the killing of Soleimani, which has been under heightened scrutiny by Democrats and some Republicans.

Trump administration officials maintain Soleimani was planning "imminent attacks" against US forces and sites. It also came after an attack on an Iraqi military base in which an American contractor was killed.

Pompeo acknowledged last week that the US did not know "precisely" when or where the imminent attacks would take place.

"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.

Esper contradicts Trump

In an interview that aired on Friday, Trump told Fox News that he believed four embassies, including the US Embassy in Baghdad, would be targeted, but he did not cite any evidence to back up that claim.

On Sunday, US Defense Secretary Mark Esper contradicted Trump, saying he did not know of any evidence of specific attacks being planned against four embassies.

"The president didn't say there was a tangible - he didn't cite a specific piece of evidence," Esper told CBS's Face the Nation programme.

Asked about whether there was any piece of evidence, Esper said: "I didn't see one with regard to four embassies. What I'm saying is, I share the president's view that probably - my expectation was they were going to go after our embassies."

While most politicians agree that Soleimani was and had been a general threat to the US, Democrats and at least two Republicans have questioned whether there was an "imminent threat" and in turn whether the administration acted legally in ordering the strike without getting congressional approval.

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".

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.

According to 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.