U S. President Donald Trump doubled down Monday on his threat to wreck Iran's civilian infrastructure, warning U.S. 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 U.S.-Israeli air assault on Tehran, the U.S. 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.

Israeli Defense Minister Israel Katz announced on Monday that Israel had conducted a "powerful strike" on Iran's largest petrochemical complex, with the operator saying it was assessing the damage from the attacks.

The complex in the Pars region services Iran's South Pars natural gas field -- the largest known gas reserve in the world -- which it shares with Qatar, and which Israel had previously hit last month.

U.S. President Donald Trump said Monday the United States has studied a proposal for a 45-day ceasefire in the Iran war, a move he called a "very significant step" in the conflict.

"It's a significant proposal, it's a significant step. It's not good enough, but it's a very significant step," Trump told reporters at the White House, adding that intermediaries "are negotiating now."

Iran has rejected a proposed truce in its war with the United States and Israel, state media reported Monday, despite a stark threat by U.S. President Donald Trump to destroy its vital infrastructure.

The U.S., Iran and a group of regional mediators are discussing the terms for a potential 45-day ceasefire that could lead to a permanent end to the war, four U.S., Israeli and regional sources with knowledge of the talks told U.S. news portal Axios.

Iran's Revolutionary Guards said they are completing preparations to enforce new operating conditions in the Strait of Hormuz, which has been all but shut since the war with the United States and Israel began.

U.S.-Israeli strikes killed on Monday the intelligence chief of Iran's Revolutionary Guards, the Guards said.

Iran launched fresh attacks across the Middle East on Monday and threatened "devastating" retaliation, after U.S. President Donald Trump's expletive-laced warning that Tehran must reopen the Strait of Hormuz.

U.S. President Donald Trump on Sunday appeared to extend by 24 hours his deadline for Iran to make a deal to reopen the Strait of Hormuz or face devastating infrastructure attacks.

U.S. President Donald Trump said Sunday he believes there is a "good chance" of making a deal with Iran on Monday, ahead of his deadline for Tehran to reopen the Strait of Hormuz or face heavy bombing.