Trump risks violating international law if he makes good on the threats, UN spokesman Stephane Dujarric told reporters Monday. He said even if the US has a military objective in attacking non-military targets, the risk of harm to civilians could make the strikes illegal.

“We were alarmed by the rhetoric seen in that social media post that threatened American attacks on power plants, bridges, and other infrastructure, should Iran not agree to a deal,” Dujarric said. That was a reference to Trump’s expletive-laden social media post on Sunday.

Dujarric said Guterres has asked the US and Iran to abide by international law and that civilian infrastructure is to be left alone. Iran has struck civilian targets in Israel and several Persian Gulf countries during the course of the war, which began Feb 28.

Guterres has generally sought to refrain from directly criticising the US or other member states. But he’s also urged the US and Israel to end the conflict and for Iran to cease attacks on its neighbours.

Last week, he called for freedom of navigation and the protection of civilians and civilian infrastructure, including nuclear installations, with the region “on the edge of a wider war”.

US forces have already carried out at least one assault on a non-military target, a bridge connecting Tehran to the city of Karaj.

Trump repeated his threats at a press conference on Monday, saying that Iran would “have no bridges” if it fails to meet a deadline he’s set for 8pm Eastern time on Tuesday to reopen the Strait of Hormuz. He said the US has a plan to destroy every power plant and every bridge in the country within four hours “if we wanted to”.

Asked if he was concerned bombing power plants and bridges could amount to war crimes, Trump said, “No, not at all.”