Iran's Ambassador to the United Nations (UN) says the United States' killing of Maj Gen Qassem Soleimani was an "act of war" and "military action" will be responded to with "military action".
On Friday (NZ time), at the direction of US President Donald Trump, the leader of Iran's infamous Quds Force was killed in an airstrike at Iraq's Baghdad International Airport. Militia leader Abu Mahdi al-Muhandis was also killed in the attack.
Soleimani was considered one of Iran's most powerful people, with close ties to Supreme Leader Ayatollah Ali Khamenei. The Quds Force was part of the Iranian Revolutionary Guard Corps and has been instrumental in many of Iran's foreign operations.
Commentators have suggested his death is the equivalent of killing the vice-president of the US, and the death has sent shockwaves around the globe.
Speculation of a regional war starting has been sparked, and speaking to CNN, Iran's Ambassador to the UN, Majid Takht Ravanchi, did nothing to put those concerns to rest.
"It was an act of war on part of the United States against the Iranian people," Ravanchi said.
The ambassador said Friday's airstrike shifted the situation between the United States and Iran from an economic conflict to one involving the military. He said the division had previously escalated when Trump decided in May 2018 to withdraw from the Iranian nuclear deal and apply sanctions to the Middle East nation.
"[Soleimani's death began] a new chapter that is tantamount to opening a war against Iran."
He echoed Khamenei in saying there would be revenge for the attack, promising justice for the Iranian people.
"Last night, they started a military war by an act of terror against one of our top generals. What else can we expect Iran to do? We cannot just remain silent, we have to act and we will act.
"The response for a military action is a military action," he said, although he refused to provide details of how that would play out.
Iran's response has prompted dozens of US citizens in the Iraqi oil city of Basra to flee, while the US Embassy in Iraq has told Americans to "depart Iraq immediately" and not approach the embassy.
Sticking to similar lines to what the Pentagon said in a statement following the airstrike, Trump said on Saturday that the decision to strike was to "stop a war".
"Soleimani was plotting imminent and sinister attacks on American diplomats and military personnel but we caught him in the act and terminated him," the president said.
"We took action last night to stop a war. We did not take action to start a war."
But Ravanchi said Trump was doing anything but, rejecting claims of an imminent attack on the United States.
"If they have evidence, they should show it. They should provide the evidence. I am sure they do not have any evidence that can be proven in a court," the ambassador said.
"What they are doing against the Iranians is exactly to put lots of pressure on the Iranian people to stand up and that is in contravention of US obligations based on international law."
Roughly 3000 additional soldiers are being sent to the Middle East following Friday's airstrike.
The White House hasn't specifically stated where Soleimani was planning attacks, but Reuters, citing a senior official, said there were attacks plotted against US diplomats in Iraq, Lebanon, Syria and elsewhere in the Middle East.
Democrats have criticised Trump's direction to strike, saying it wasn't educated and was ordered without congressional permission. However, sources have told CNN there was a consensus within the president's national security cabinet that based upon intelligence, action was needed. Three sets of White House lawyers also vetted the plan and said Trump did not need authorisation.
The Trump-directed attack comes after a week of heightened tension between Iran and the US. Last week, an America contractor in Iraq was killed in a rocket attack which the US blamed on Iranian-backed militia. In response, the US killed 25 Kataeb Hezbollah fighters, leading militias to attack the US Embassy in Baghdad.
An additional airstrike hit in the Iraq area of Taji on Saturday, killing at least six people. It's unclear who is responsible, with the US denying involvement.