Donald Trump has given Iran a deadline and threatened to send the country back to the "Stone Ages" if it does not comply by attacking power plants and bridges.
Crucially, Mr Trump has said the US would hit civilian infrastructure, something that is widely considered a war crime under international law, which sets out what is and isn't permitted during war.
The Geneva Convention, which the UK is a signatory to, but the US is not, states, "civilian objects shall not be the object of attack or of reprisals".
However, the US president said in a news conference on Monday that "the entire country" could be taken out in "one night, and that night might be tomorrow".
He added: "We're giving them till tomorrow, 8pm EST (1am UK time), and after that, they're going to have no bridges, they're going to have no power plants. Stone ages."
When asked, Mr Trump said he was "not at all" concerned that such an act could be classified as a war crime, before adding, "I hope I don't have to do it".
In a Truth Social post on Tuesday, the president went further, saying: "A whole civilisation will die tonight, never to be brought back. I don't want that to happen, but it probably will."
"Any targeting of civilian infrastructure... is illegal"
The possibility of targeting civilian infrastructure has been condemned by European leaders, including EU Council president Antonio Costa.
On Monday, Mr Costa wrote that "Any targeting of civilian infrastructure, namely energy facilities, is illegal and unacceptable".
The possibility of concerted US strikes against civilian infrastructure marks a significant departure from Washington's previous stance on war crimes and what constitutes them.
Four years ago, it was the US that was accusing Russia of war crimes over the targeting of civilian infrastructure in Ukraine, with president Joe Biden calling Russian President Vladimir Putin a "war criminal".
Despite Russia not being a signatory to the Geneva Conventions, alleged Russian actions in Ukraine still led to the International Criminal Court (ICC) issuing an arrest warrant for Mr Putin.
The Kremlin said at the time that Russia, which does not recognise the ICC, found the questions raised by the court "outrageous and unacceptable".
In the past, those found guilty of war crimes, such as former Bosnian Serb general Ratko Mladic, dubbed the "Butcher of Bosnia", have been sentenced to life in prison.
Mr Costa has said that the same principle of not targeting civilian infrastructure "applied everywhere," in the same way that it "applies to Russia's war in Ukraine".
The head of the UN, Antonio Guterres, has openly raised the idea that such an attack, by any party, could be counted as war crimes.
In an interview with Politico in late March, Mr Guterres said: "If there are attacks either on Iran or from Iran on energy infrastructure, I think that there are reasonable grounds to think that they might constitute a war crime."
Previous power threats
Iran, Israel and the US have all been attacking energy infrastructure since the outbreak of the current conflict.
Responding to those attacks in March, Heba Morayef, Amnesty International's regional director, said: "There is a substantial risk such attacks would violate international humanitarian law and, in some cases, could amount to war crimes".
However, a coordinated campaign targeting power plants, as the US has threatened, would be a significant escalation.
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This is not the first time that Mr Trump has threatened to hit Iranian power plants.
On 22 March, the president wrote on Truth Social that the US would "hit and obliterate" Iranian power plants if Tehran did not reopen the Strait of Hormuz within 48 hours.
Iran responded by saying it would attack crucial infrastructure across the Gulf in response to any US attack, including desalination plants.
On Sunday, Kuwait accused Iran of launching an attack which put a water desalination station out of service - such moves may also pass the threshold of being deemed a war crime.
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