Lieutenant General Qassem Soleimani, the 62-year-old commander of the Quds Force of the Islamic Revolution Guards Corps (IRGC), was killed in the US airstrike in Baghdad overnight. The attack, ordered by President Donald Trump, sent tensions between the United States and Iran soaring, with Iranian officials promising revenge, Reuters reported.

Iran’s Foreign Minister Mohammad Javad Zarif told state TV on Friday that Iran will take legal measures at the international level to hold the United States accountable for the assassination of General Soleimani.

“It was clearly a terrorist action … Iran will launch various legal measures at the international level to hold America to account for Soleimani’s assassination,” Zarif said.

As Republican and Democratic lawmakers sparred over the wisdom of the attack, some legal experts questioned whether Trump, unlike his ‘self-defense’ justification, had the legal authority to target Soleimani on Iraqi soil without the permission of Iraq’s government, and whether it was legal under international and US law.

Iraq’s prime minister said Washington had with the attack violated a deal for keeping US troops in his country, and several Iraqi political factions united in a call for American troops to be expelled.

The UN Charter generally prohibits the use of force against other states but there is an exception if a state gives consent to the use of force on its territory. Legal experts said the absence of consent from Iraq makes it difficult for the United States to justify the killing.

Yale Law School professor Oona Hathaway, an international law expert, said on Twitter that the available facts “do not seem to support” the assertion that the strike was an act of self-defense, and concluded it was “legally tenuous under both domestic and international law.”

The Pentagon claimed targeting Soleimani was aimed at deterring “future Iranian attack plans,” while Trump claimed the Iranian general was targeted because he was planning “imminent and sinister” attacks on US diplomats and military personnel and did it as ‘self-defense’ act.

Scott Anderson, a former legal adviser to the US Embassy in Baghdad under Trump’s predecessor, Barack Obama, said Trump’s justification so far under international law is questionable.

A strategic framework agreement signed in 2008 between Washington and Baghdad called for close defense cooperation to deter threats to Iraqi “sovereignty, security and territorial integrity,” but prohibited the United States from using Iraq as a launching point for attacks on other countries.

Under historic norms of international law, a country can defend itself preemptively if it acts out of necessity and responds proportionally to the threat.

Agnes Callamard, the UN special rapporteur on extra-judicial executions, questioned whether the attack met this threshold.

The targeting of Soleimani “appears far more retaliatory for past acts than anticipatory for imminent self-defense,” she said. “Lawful justifications for such killings are very narrowly defined and it is hard to imagine how any of these can apply to these killings.”

Democratic lawmakers called on Trump to provide details about the imminent threat that he said Soleimani represented.

“I believe there was a threat, but the question of how imminent is still one I want answered,” Senator Mark Warner, the Democratic vice-chair of the Senate Intelligence Committee, told Reuters.

Other critics raised questions about Trump’s authority to kill Soleimani under US law, and whether he should have acted without first notifying Congress.