The United States' killing of Iran's most prominent military commander, Qassem Soleimani, is virtual to starting a war and "the response for a military action is a military action," Iran's UN ambassador said on Friday.
Ambassador Majid Takht Ravanchi said in an interview with CNN that by "assassinating" Soleimani, the United States had entered a new stage after starting an "economic war" by imposing tough sanctions on Iran in 2018.
"So that was ... a new chapter which is tantamount to opening a war against Iran," Ravanchi said.
Ravanchi, echoing Iranian leaders, said there would be harsh revenge. "The response for a military action is a military action," he said.
Earlier on Friday, the ambassador told the UN Security Council and Secretary-General Antonio Guterres that Iran reserves the right to self-defense under international law.
In a letter, Ravanchi said the killing of Soleimani "is an obvious example of State terrorism and, as a criminal act, constitutes a gross violation of the fundamental principles of international law, including, in particular ... the Charter of the United Nations."
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The United States killed Soleimani in an overnight attack in Iraq authorized by President Donald Trump. A senior Trump administration official said Soleimani had been planning imminent attacks on U.S. personnel in the Middle East.
The United States could seek to justify killing Soleimani under Article 51 of the UN Charter, which covers an individual or collective right to self-defense against armed attack.
Under Article 51, countries are required to "immediately report" to the 15-member Security Council any measures taken in exercising the right of self-defense. The United States used Article 51 to justify taking action in Syria against Islamic State militants in 2014.
Diplomats said no such letter had yet been received from Washington on the killing of Soleimani.
Guterres is deeply concerned by the recent rise in tensions in the Middle East, his spokesman, Farhan Haq, said in a statement earlier on Friday.
"This is a moment in which leaders must exercise maximum restraint. The world cannot afford another war in the Gulf," Haq said.
Some legal experts and a senior UN rights investigator questioned whether Trump 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 U.S. law.
Iraq's prime minister said Washington had with the attack violated a deal for keeping U.S. 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 said targeting Soleimani was aimed at deterring "future Iranian attack plans," while Trump said the Iranian general was targeted because he was planning "imminent and sinister" attacks on U.S. diplomats and military personnel.
Robert Chesney, a national security law expert at the University of Texas at Austin School of Law, said the administration's best argument on the UN Charter issue is self defense. "If you accept that this guy was planning operations to kill Americans, that provides the authority to respond," he said.
Scott Anderson, a former legal adviser to the U.S. Embassy in Baghdad under Trump's predecessor, Barack Obama, said Trump's justification so far under international law is questionable, but he could try to argue that the Iraqi government was either unwilling or unable to deal with the threat posed by Soleimani, giving the United States the right to act without Iraq's consent.
U.S. forces in Iraq had been fighting Islamic State, and about 5,000 troops remain, most of them in an advisory capacity.
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 U.S. law, and whether he should have acted without first notifying Congress.
Legal experts noted that recent U.S. presidents from both parties have taken an expansive view of their unilateral ability to preemptively engage in force, including through targeted killings, a view bolstered by executive branch lawyers in successive administrations.
In the case of Soleimani, the administration's self-defense arguments may hinge on disclosing specific knowledge of his imminent plans to attack Americans.
Self-defense could allow the administration to act without having to first notify Congress or act under a prior congressional authorization for the use of military force, Chesney said.
Democratic lawmakers did not defend Soleimani, who U.S. officials have said is responsible for the deaths of hundreds of Americans, but they called on Trump to consult with Congress going forward.
"This administration, like all others, has the right to act in self-defense," said Rep. Elissa Slotkin, a former Central Intelligence Agency analyst who worked in Iraq focusing on Iranian-backed militias. "But the administration must come to Congress immediately and consult."