The United States has failed to provide sufficient evidence of an ongoing or imminent attack against its interests to justify the strike on Soleimani's convoy as it left Baghdad airport, said Agnes Callamard, the UN special rapporteur on extrajudicial, summary or arbitrary executions, Reuters reported.
The attack violated the UN Charter, Callamard wrote in a report calling for accountability for targeted killings by armed drones and for greater regulation of the weapons.
"The world is at a critical time, and possible tipping point, when it comes to the use of drones. ... The Security Council is missing in action; the international community, willingly or not, stands largely silent," Callamard, an independent investigator, said.
Callamard is due on Thursday to present her findings to the Human Rights Council, giving member states a chance to debate what action to pursue. The United States is not a member of the forum, having quit two years ago.
"General Soleimani was in charge of Iran 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," Callamard wrote in the report.
The drone strike was the first known incident in which a nation invoked self-defense as a justification for an attack against a state actor in the territory of a third country, Callamard added.
On January 3, the US assassinated Soleimani, the commander of the Quds Force of Iran's Islamic Revolution Guards Corps, and Abu Mahdi al-Muhandis, the second-in-command of Iraq’s Popular Mobilization Units (PMU), and a group of their companions in Baghdad.
The operation was conducted with the authorization of US President Donald Trump.
Iran retaliated with a missile attack on US forces were stationed based in Iraq.
Iran has issued an arrest warrant for Trump and 35 others over Lieutenant General Soleimani's assassination and has asked Interpol for help, Tehran prosecutor Ali Alqasimehr said on June 29.