Washington DC - The US military said Sunday it would begin a blockade of all Iranian ports within hours on President Donald Trump's orders after peace talks in Pakistan collapsed.

The announcement came despite a ceasefire agreed on Wednesday aimed at pausing the six-week war started by Israel and the US until April 22.

"The blockade will be enforced impartially against vessels of all nations entering or departing Iranian ports and coastal areas, including all Iranian ports on the Arabian Gulf and Gulf of Oman," US Central Command said in a statement, adding that it would begin at 10 AM ET Monday.

The statement said US forces would not impede vessels transiting the Strait of Hormuz to and from non-Iranian ports, and that further instructions would follow.

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Iran's Revolutionary Guards had earlier warned they had full control of traffic through Hormuz and would trap any challenger "in a deadly vortex."

In a rambling social media post earlier, Trump bafflingly ordered the blockade of an already blockaded waterway.

"Effective immediately, the United States Navy, the Finest in the World, will begin the process of BLOCKADING any and all Ships trying to enter, or leave, the Strait of Hormuz," Trump said. "Any Iranian who fires at us, or at peaceful vessels, will be BLOWN TO HELL!"

"We are not going to let Iran make money on selling oil to people they like or not selling it to people they don't like. It's going to be all or nothing," Trump raged on Fox News.

Iran's parliament speaker Mohammad Bagher Ghalibaf, who led Tehran's delegation in Pakistan, said on returning home that the country would "not bow to any threats" from Washington, while navy chief Shahram Irani called Trump's blockade threat "ridiculous."

Tehran has already been restricting traffic through the strait – a key route for global oil and gas shipments – while allowing some vessels serving friendly countries such as China to pass. Further jamming oil transit through an all-out blockade is could have devastating effects on the global economy.