Efforts to build a more lasting peace in the region likewise appeared to be on shaky ground, as Iran said it would not participate in a second round of negotiations the US had hoped to kick off before the ceasefire expires on Tuesday.
The US 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 supply.
The US military said it fired on an Iranian-flagged cargo ship headed towards Iran's Bandar Abbas port on Sunday after a six-hour stand-off, disabling its engines.
US marines then rappelled from helicopters onto the vessel, US Central Command said.
"We have full custody of their ship, and are seeing what's on board!" President Trump wrote on social media.
Iran's military said the ship had been travelling from China.
"We warn that the armed forces of the Islamic Republic of Iran will soon respond and retaliate against this armed piracy by the US military," a military spokesperson said, according to state media.
Oil prices jumped more than five per cent and stock markets wobbled as traders fretted the ceasefire would collapse and traffic in and out of the Gulf would remain at a bare minimum.
Iranian state media reported Tehran had rejected new peace talks, 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," Iran's 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 Iran the US would destroy every bridge and power plant in Iran if Tehran rejected his terms.
Iran has vowed to hit Gulf Arab neighbours' power stations and desalination plants if the US attacks its civilian infrastructure.
Trump said his envoys would arrive in Islamabad on Monday evening, one day before a two-week ceasefire ends.
A White House official told Reuters the US delegation would be headed by Vice President JD Vance, who led the war's first peace talks a week ago, and would include Trump's envoy Steve Witkoff and son-in-law Jared Kushner.
But Trump told ABC News and MS Now Vance would not go.
Pakistan, which has served as the main mediator, appeared to be preparing for the talks.
Two giant US C-17 planes carrying security equipment and vehicles landed at an air base on Sunday in preparation for the US delegation's arrival, Pakistani security sources said.
Municipal authorities in the Pakistani capital of Islamabad halted public transport and heavy goods traffic through the city.
Barbed wire was rolled out near the Serena Hotel, where last week's talks were held, and guests were told to leave.
Iran's parliament speaker Mohammad Baqer Qalibaf, who has led the republic's side in the talks, had earlier said 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 Washington's negotiating team is pushing for a swift, superficial deal that would require months or years of complex follow-on talks.
Now in its eighth week, the war has created the most severe shock to global energy supplies in history, with oil prices surging.
Thousands of people have been killed by US-Israeli strikes on Iran and in an Israeli invasion of Lebanon conducted in parallel since the war began on February 28.
Iran responded to the attacks with missiles and drones against Israeli and nearby Arab countries that host US bases.
The Islamic Republic executed two men convicted of co-operating with Israel's Mossad intelligence service and planning attacks inside the country, the judiciary's news outlet Mizan reported on Sunday.
