The new US assault across multiple Iranian cities came as efforts to negotiate an end to the war again appeared stuck, with Iran insisting it would maintain its chokehold on the Strait of Hormuz, which has disrupted global energy supplies and sent oil prices higher.

It was the third time this week that back-and-forth strikes have tested a two-month shaky ceasefire.

Trump has urged Iran to sign a deal to end the war and suggested earlier this week that an agreement could be reached in days.

Iran's United Nations envoy said the US should refrain from threats of force if it wants a deal.

"Iran has never negotiated under threats and pressure and will never submit to pressure or question," Ambassador Amir Saeid Iravani told the UN Security Council on Wednesday.