Trump, who has previously offered shifting timelines and objectives for the war, said in a televised speech on Wednesday the war could escalate if Iran did not give in to Washington's terms, with strikes on its energy and oil infrastructure possible.
More than 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 in March said 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," he said.
A leading US Muslim advocacy group warned Trump's rhetoric during the war, including his threat to strike Iran "back to the Stone Ages," has been "dehumanising".
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 of people and displaced millions.
