"It takes a special kind of intellectual dishonesty to issue a report condemning the United States for acting in self-defense while whitewashing General Soleimani's notorious past as one of the world's deadliest terrorists," State Department spokeswoman Morgan Ortagus said.

"This tendentious and tedious report undermines human rights by giving a pass to terrorists and it proves once again why America was right to leave" the UN Human Rights Council, she said.

Agnes Callamard, the UN special rapporteur on extrajudicial, summary or arbitrary executions, concluded Tuesday that Soleimani's killing in January at the Baghdad airport violated the UN charter.

The US had provided no evidence that an imminent attack against US interest was being planned, she wrote, calling it an "arbitrary killing."

"Soleimani was in charge of Iran's military strategy, and actions, in Syria and Iraq. But absent an actual imminent threat to life, the course of action taken by the US was unlawful," she wrote.

The independent rights expert does not speak for the United Nations but reports her findings to it.

She will present her findings on Thursday to the Human Rights Council, from which US President Donald Trump withdrew in 2018, pointing in part to alleged bias against Israel.

Callamard, expanding on her report on Twitter on Wednesday, noted the rapid proliferation of drones around the world.

She warned that states could not be allowed to distort the concept of an "imminent" attack.

"For the first time, a State drone targeted a high-level official of another state on the territory of a third one," she wrote.

"The international community must now confront the very real prospect that States may opt to 'strategically' eliminate high-ranking military officials outside 'known' wars," she wrote.

Trump ordered the drone strike, which also killed Iraqi commander Abu Mahdi al-Muhandis, after escalating tensions between the United States and Iran, against which Trump has imposed sweeping unilateral sanctions.

Shiite paramilitary groups backed by Iran had been blamed for a series of attacks on bases housing US troops and later for trashing the US embassy in Baghdad.

Trump administration officials later faced grilling by lawmakers, mostly from the rival Democratic Party, who said there was no evidence of an imminent attack that would legally justify killing a top Iranian official.

Soleimani headed Iran's elite Revolutionary Guards' Quds Force which carries out operations outside of Iran including in support of Syrian President Bashar al-Assad.