By Parisa Hafezi and Hatem Maher

DUBAI, April 20 (Reuters) - A ceasefire between the United States and Iran appeared in jeopardy on Monday after the U.S. said it had seized an Iranian cargo ship that tried to run its blockade and Tehran vowed to retaliate, refusing for now to join new peace talks.

Iranian Foreign Ministry spokesman Esmaeil Baghaei said Washington has shown it was "not serious" about pursuing the diplomatic process and Tehran would not change its clearly stated demands, adding that it did not believe in deadlines or ultimatums when safeguarding national interests.

The U.S. had hoped to start negotiations in Pakistan shortly before the two-week ceasefire expires, with sweeping security preparations under way in Islamabad, but Baghaei said the U.S. was "insisting on some unreasonable and unrealistic positions".

A senior Iranian source told Reuters the continuation of the U.S. blockade on Iranian ports was undermining the prospect of peace talks, and Tehran's "defensive capabilities", including its missile programme, were not open to negotiation.

A Pakistani security source said Pakistan's key mediator, Field Marshal Asim Munir, had told U.S. President Donald Trump the blockade was an obstacle to talks, and that Trump had replied that he would consider the advice.

U.S. MARINES BOARD IRANIAN VESSEL

The U.S. has maintained a blockade of Iranian ports, while Iran has lifted and then reimposed its own blockade on marine traffic passing through the Strait of Hormuz, which typically handles roughly one-fifth of the world's oil and liquefied gas supply.

Oil prices rose more than 6% and stock markets wobbled as traders fretted that the ceasefire would collapse and traffic in and out of the Gulf would remain at a bare minimum.

The U.S. military said it had fired on an Iranian-flagged cargo ship headed towards Iran's Bandar Abbas port on Sunday after a six-hour standoff, disabling its engines. U.S. Central Command released video showing Marines descending ropes from helicopters onto the vessel.

Iran's military said the ship had been travelling from China and accused the U.S. of "armed piracy", according to state media. They said they were ready to confront U.S. forces over the "blatant aggression", but were constrained by the presence of crew members’ families on board.

China expressed concern over the "forced interception", and a Chinese foreign ministry spokesperson urged relevant parties to abide by the ceasefire agreement in a responsible manner.

Tehran refused new peace talks for now, citing the ongoing blockade, threatening rhetoric, and Washington's shifting positions and "excessive demands".

"One cannot restrict Iran's oil exports while expecting free security for others," First Vice President Mohammadreza Aref wrote on social media. "The choice is clear: either a free oil market for all, or the risk of significant costs for everyone."

Trump earlier warned that the U.S. would destroy every bridge and power plant in Iran if it rejected his terms, continuing a recent pattern of such threats.

Iran has said that if the United States were to attack its civilian infrastructure, it would strike power stations and desalination plants in its Gulf Arab neighbours.

PREPARING FOR TALKS THAT MIGHT NOT HAPPEN

Trump said his envoys would arrive in Islamabad on Monday evening, one day before the two-week ceasefire ends.

A White House official told Reuters the U.S. delegation would be headed by Vice President JD Vance, who led the U.S. delegation to the first round of talks a week ago, and also include Trump's envoy Steve Witkoff and son-in-law Jared Kushner. But Trump told ABC News and MS Now that Vance would not go.

Pakistan, which has served as the main mediator, was still gearing up for the talks.

Nearly 20,000 police, paramilitary and army personnel have been deployed across the capital Islamabad, a government and a security official said.

Iran's parliament speaker, Mohammad Baqer Qalibaf, who has led Iran's side in the talks, said on Saturday the two sides had made progress but were still far apart on nuclear issues and the Strait.

European allies, repeatedly criticised by Trump for not aiding his war effort, worry that Washington's negotiating team is pushing for a swift, superficial deal that would require months or years of technically complex follow‑on talks.

Now in its eighth week, the war has created the most severe shock to global energy supplies in history, sending oil prices surging because of the de facto closure of the strait.

Thousands of people have been killed by U.S.-Israeli strikes on Iran and in an Israeli invasion of Lebanon conducted in parallel since the war began on February 28, where a truce is also currently in place.

Iran responded to the attacks with missiles and drones against Israel and nearby Arab countries that host U.S. bases.

(Reporting Reuters bureaus; Writing by Andy Sullivan and John Geddie; Editing by Lincoln Feast, Alexandra Hudson)

Posted by: AAngeles/NET25 News