Washington DC - Washington DC - President Donald Trump doubled down Monday on his threat to wreck Iran's civilian infrastructure, warning US forces could destroy every bridge and power plant in the country within four hours and that a truce proposal from international mediators was not yet enough.

Five weeks into the Middle East war triggered by a joint US-Israeli air assault on Tehran, the US leader has demanded that Iran reopen the Strait of Hormuz to international shipping by midnight GMT on Tuesday, or face a newly devastating round of bombing.

Iran's virtual blockade of Hormuz has sent oil and gas prices soaring and pushed countries around the world to enact measures to contain the fallout.

Both Trump and Iran have said that a proposal touted by international mediators for a 45-day ceasefire is not yet ready, and in a Washington press conference, the US president dialed up his warlike rhetoric once again.

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"We have a plan – because of the power of our military – where every bridge in Iran will be decimated by 12 o'clock tomorrow night, where every power plant in Iran will be out of business, burning, exploding, and never to be used again," Trump said.

"I mean complete demolition by 12 o'clock, and it'll happen over a period of four hours – if we wanted to."

Trump had earlier accepted the ceasefire plan was a "significant proposal," but went on to say it was not good enough. Iranian state media quotes officials stating that Tehran too "has rejected a ceasefire and insists on the need for a definitive end to the conflict."

Trump said intermediaries "are negotiating now" on improving the ceasefire proposal, which US media reported was being mediated by Pakistan, Egypt and Turkey.

Iran's military said it would "continue the war as long as the political authorities see fit."

But talk of a ceasefire came as the US and Israel were striking targets across Iran, including major petrochemical facilities, and as Iran continued missile and drone attacks around the region.

Earlier Monday, Israeli strikes had hit major Iranian petrochemical facilities including in Asaluyeh on the Gulf coast, the country's biggest, and another outside Shiraz in central Iran.

Israel's military said it had also struck Iranian air force targets including planes and helicopters at airports in Tehran and elsewhere.