Dozens of international law experts in the US have signed an open letter saying that American strikes on Iran may amount to war crimes, after US President Donald Trump reiterated his threats this week to strike Iran's power and desalination plants.

Trump, who has previously offered shifting timelines and objectives for the war, said in a televised speech on Wednesday that the war could escalate if Iran did not give in to Washington's terms, with strikes on its energy and oil infrastructure possible.

Over 100 international law experts in the US, including from schools like Harvard, Yale, Stanford, and the University of California, said in the letter released on Thursday that the conduct of US forces and statements by senior US officials "raise serious concerns about violations of international human rights law and international humanitarian law, including potential war crimes."

The letter particularly noted a mid-March comment from Trump where he said the US may conduct strikes on Iran "just for fun." It also cited comments from Pentagon chief Pete Hegseth from early March in which he said the US does not fight with "stupid rules of engagement."

The letter was published on the website of the Just Security policy journal.

The experts said they were "seriously concerned about strikes that have hit schools, health facilities, and homes," noting a strike on a school in Iran on the war's first day.

The US military said in March it elevated the investigation into a devastating February 28 strike on an Iranian girls' school after media reports revealed ​the probe showed US forces were likely responsible. The Iranian Red Crescent says 175 were killed in the strike.

Trump on Wednesday threatened to strike Iran "extremely hard."

"We are going to hit ​them extremely ​hard over the next two ‌to ⁠three weeks. We are going to bring ​them ​back ⁠to the Stone Ages, where ​they belong," ​Trump said.

A leading US Muslim advocacy group warned that Trump's rhetoric during the war, including his threat to strike Iran "back to the Stone Ages," has been "dehumanizing."

The war began on February 28 when the US and Israel attacked ⁠Iran. Tehran responded by launching its own attacks on Israel and Gulf states with US bases. Joint ​US-Israeli strikes in Iran and Israeli attacks in Lebanon have killed thousands and displaced millions.