In an interview with CNN on Friday, Majid Takht-Ravanchi said by assassinating General Soleimani, the United States had entered a new stage after starting an "economic war" by imposing sanctions on Iran in 2018.

"So that was ... a new chapter which is tantamount to opening war against Iran," Ravanchi said.

Stressing that there would be harsh revenge, he said, "The response for a military action is a military action”.

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 assassination of the top commander "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."

Major General Soleimani, the deputy head of Iraq’s Hashd al-Shaabi (PMU), and a number of their entourage were killed in a strike by American drones near Baghdad International Airport in the early hours of Friday.

The White House and the Pentagon confirmed the assassination of General Soleimani in Iraq, saying the attack was carried out at the direction of US President Donald Trump.

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 (Persian) Gulf," Haq said.