US President Donald Trump clearly had the authority to kill Qassem Soleimani, United States Attorney General William Barr said on Monday, adding that the White House consulted with his Department of Justice before the strike on the Iranian commander.

Barr told reporters that Soleimani was a "legitimate military target" and the strike was a "legitimate act of self-defence".

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Barr did not cite any specific evidence, but said, "We had a situation where the Iranians had already embarked on a series of escalating violent action taken against our allies, taken against the American people, our troops, with the avowed purpose of driving us out of the Middle East."

He added: "The Department of Justice was consulted and frankly, I don't think it was a close call."

Since the January 3 strike that killed Soleimani, critics have questioned the Trump administration's assertion that Soleimani was planning an imminent attack against the US. They have also questioned the timing of the strike, coordination within the administration and Trump's decision not to notify Congress, which holds the power to declare war, of his plans.

"The general in charge of these efforts, Soleimani, was clearly a legitimate military target. We had a very brief window of time to carry out the attack," Barr said.

"This was a legitimate act of self-defence because it disrupted ongoing attacks that were being conducted - a campaign against the Americans - and it re-established deterrence," he added, without elaborating.

Earlier on Monday, Trump brushed aside concerns, tweeting it "doesn't really matter" if a threat was impending.

"The Fake News Media and their Democrat Partners are working hard to determine whether or not the future attack by terrorist Soleimani was 'imminent' or not, & was my team in agreement," Trump tweeted.

"The answer to both is a strong YES., but it doesn't really matter because of his horrible past!"

The Fake News Media and their Democrat Partners are working hard to determine whether or not the future attack by terrorist Soleimani was “imminent” or not, & was my team in agreement. The answer to both is a strong YES., but it doesn’t really matter because of his horrible past! — Donald J. Trump (@realDonaldTrump) January 13, 2020

Last week, Trump said in an interview that Iran had been poised to attack four American embassies, but on Sunday, defence secretary, Mark Esper, said he did not see specific evidence that Iran was planning an attack.

"What the president said was that there probably could be additional attacks against embassies. I shared that view," Esper said. "The president didn't cite a specific piece of evidence."

When pressed on whether intelligence officers offered concrete evidence on that point, Esper said: "I didn't see one with regards to four embassies."

Democrats and some Republicans in Congress have questioned the justification of the attacks and said they have not been given adequate, detailed briefings.

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 Senate will debate a similar resolution this week.

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.