Concerns are mounting over the durability of the ceasefire between the United States and Iran after Washington said it had seized an Iranian cargo vessel accused of breaching its maritime blockade, prompting a warning of retaliation from Tehran.

According to the U.S. military, forces opened fire on an Iranian-flagged ship following a prolonged standoff in waters near the Strait of Hormuz, disabling its engines before troops boarded the vessel. U.S. Central Command said marines descended from helicopters and took control of the ship.

U.S. President Donald Trump confirmed the operation in a social media post, saying American forces had “full custody” of the vessel and were inspecting its cargo.

Iranian authorities condemned the seizure, describing it as “armed piracy.” A military spokesperson, quoted by state media, warned that Iran’s armed forces would respond. Officials said the ship had been en route from China to the Iranian port of Bandar Abbas.

Ceasefire and talks in doubt

The incident has cast doubt on efforts to sustain a fragile ceasefire set to expire on Tuesday. Hopes for a broader agreement also appeared uncertain after Iranian state media reported that Tehran would not take part in a planned second round of negotiations.

Iranian officials cited ongoing U.S. blockade measures, escalating rhetoric, and what they described as shifting demands from Washington. First Vice President Mohammadreza Aref said Iran would not accept restrictions on its oil exports while others benefited from open energy markets.

“If Iran’s oil is restricted, others should not expect full security,” he said in remarks carried by state media.

Preparations for uncertain negotiations

Despite the tensions, diplomatic activity is continuing. U.S. envoys are expected to travel to Islamabad, which has been acting as a key mediator. However, uncertainty remains over the composition of the U.S. delegation and whether talks will proceed as planned.

Security measures in Islamabad have been stepped up ahead of the anticipated visit, with restrictions imposed on transport and heightened protection around key venues.

Iran’s parliament speaker Mohammad Baqer Qalibaf had earlier indicated that while some progress had been made in initial discussions, significant gaps remain, particularly over nuclear issues and maritime security.

Global market impact

The developments have unsettled global markets. Oil prices rose sharply amid fears that the ceasefire could collapse, further disrupting flows through the Strait of Hormuz — a critical route for roughly one-fifth of the world’s oil supply.

The conflict, now in its eighth week, has already caused major disruptions to regional energy production. Countries including Saudi Arabia, Iraq, Kuwait and the United Arab Emirates have seen significant declines in output.

Escalating rhetoric

Tensions have been further heightened by increasingly sharp rhetoric. President Trump has warned that the United States could target key Iranian infrastructure if negotiations fail, while Iranian officials have threatened retaliatory strikes against regional energy and utility facilities if attacked.

The war, which began in late February, has triggered one of the most severe shocks to global energy markets in recent history and raised fears of broader regional escalation.

U.S. President Donald Trump has accused Iran of violating a ceasefire agreement in the Strait of Hormuz and warned of sweeping military consequences if tensions escalate further.

In a message posted on Truth Social, Trump claimed that Iranian forces “fired bullets” in the vital shipping corridor, describing the incident as a “total violation” of the ceasefire. He alleged that the gunfire was directed at a French vessel and a United Kingdom-flagged freighter, though the claims have not been independently verified.

Trump said U.S. representatives would travel to Islamabad for negotiations, indicating that diplomatic efforts are continuing despite rising tensions.

The comments come amid heightened friction between Washington and Tehran over maritime security. The Strait of Hormuz, a critical chokepoint for global oil shipments, has long been a focal point of confrontation between Iran and Western powers.

In his post, Trump also claimed Iran had moved to close the waterway, while asserting that a U.S. “blockade” had already effectively halted traffic. He argued that such a closure would primarily harm Iran, estimating losses of up to $500 million per day.

He added: “In fact, many Ships are headed, right now, to the U.S., Texas, Louisiana, and Alaska, to load up, compliments of the IRGC, always wanting to be ‘the tough guy!’.”

In his sharply worded warning, Trump said that if Iran does not accept what he described as a “fair and reasonable deal,” the United States could target key infrastructure, including power plants and bridges.

“We’re offering a very fair and reasonable DEAL, and I hope they take it because, if they don’t, the United States is going to knock out every single Power Plant, and every single Bridge, in Iran.

“NO MORE MR. NICE GUY! They’ll come down fast, they’ll come down easy and, if they don’t take the DEAL, it will be my Honor to do what has to be done, which should have been done to Iran, by other Presidents, for the last 47 years. IT’S TIME FOR THE IRAN KILLING MACHINE TO END!” he wrote.

Iran has not yet publicly responded to the remarks.

Analysts note that any escalation around the Strait of Hormuz carries significant global risks, given the route’s central role in international energy supplies.

All ships can sail through the Strait of Hormuz but this needs to be coordinated ‌with Iran’s Islamic Revolutionary Guard Corps (IRGC), a senior Iranian official told Reuters, adding that unfreezing Iranian funds was part of the deal.

Foreign Minister Abbas Araqchi wrote on X that the strait was open after a ceasefire accord was agreed in Lebanon, while U.S. President Donald Trump said he believed a deal to end the Iran war would come “soon”, although the timing remains unclear.

Hundreds ​of ships and 20,000 seafarers have remained stranded inside the Gulf waiting to pass through the key waterway, which handles about 20% of the ​world’s oil and liquefied natural gas flows.

The Iranian official said transits would be restricted to lanes that Iran deemed safe, ⁠adding that military vessels were still prohibited from crossing the strait.

“Releasing Iran’s funds was part of the agreement for reopening the strait,” the official noted, referring ​to an estimated $30 billion in frozen revenue, generated mainly from oil and gas exports, blocked amid U.S. sanctions on Tehran.

It was not immediately clear if this included or ​excluded the established Traffic Separation Scheme (TSS) lanes for entering and exiting the Gulf used by international shipping since the 1970s.

“Even U.S. vessels would be permitted, excluding military ships,” he said.

The official added that certain routes through Hormuz would remain open, but added that those would need to be determined as secure by Iran.

“Navigation would take place in coordination with Iran, and with ​authorisation from the Guards and Iran’s Ports and Maritime Organization to ensure the safety of shipping,” the official said.

Shortly after Araqchi’s statement, U.S. President Donald Trump ​posted on Truth Social: “IRAN HAS JUST ANNOUNCED THAT THE STRAIT OF IRAN IS FULLY OPEN AND READY FOR PASSAGE”.

Trump added that the U.S. military blockade of ships sailing through the ‌strait to ⁠and from Iranian ports, announced after talks with Iran last weekend in Pakistan which ended without agreement, remained in place.

Iranian state media, citing an unnamed official, said if the U.S. blockade persists, Tehran will consider it a violation of ceasefire and will re-close strait.

Iran could let ships sail freely through the Omani side of the strait without risk of attack under proposals Tehran has offered in talks with the U.S., providing a deal is clinched to prevent renewed conflict, a source briefed by ​Tehran told Reuters this week.

Iran has warned of mines placed ⁠in the strait, a threat taken seriously by ship owners, insurance underwriters, and firms sending cargo.

That threat is not fully understood and avoidance of the area by ships should be considered, a U.S. Navy advisory said on Friday.

“Status of TSS mine ​threat is not fully understood. Consider avoidance of that area,” the US Navy’s U.S. NCAGS agency said in the ​advisory sent to mariners ⁠and seen by Reuters.

It was not immediately clear whether the advisory was sent before or after the announcement about the strait being open.

Shipping industry associations said they were reviewing the situation.

“The announcement … by U.S. President Donald Trump that the Strait of Hormuz is fully open is inaccurate. The status of mine threats in the Traffic Separation Scheme ⁠is unclear,” ​said Jakob Larsen, chief safety and security officer with shipping association BIMCO.

“BIMCO believes shipping companies should ​consider avoiding the area”.

The UN’s shipping agency, the International Maritime Organization (IMO), said it was reviewing the situation.

“We are currently verifying the recent announcement related to the reopening of the Strait of Hormuz, in terms ​of its compliance with freedom of navigation for all merchant vessels and secure passage,” said IMO Secretary-General Arsenio Dominguez.