Iran’s Khatam al-Anbiya Central Headquarters announced on Saturday that the Strait of Hormuz would be closed to all maritime traffic, accusing the United States of reneging on its commitments under a newly signed memorandum of understanding (MoU) and citing ongoing Israeli military actions in southern Lebanon. In a statement, the Iranian military command said the closure was a first step in response to the “clear treachery” of Washington’s failure to enforce the accord’s opening clause—an immediate and permanent end to hostilities on all fronts—and to what it described as continuous ceasefire violations by “the Zionist entity.” Simultaneously, the Islamic Revolutionary Guard Corps Navy warned vessels not to approach the strait, threatening their safety if they did.

Reacting within hours, United States Central Command (CENTCOM) asserted that the strait remained open, with 55 commercial ships carrying more than 17 million barrels of oil transiting the waterway on Saturday alone. CENTCOM said American forces in the region were “vigilant” and “ready to ensure all aspects of the agreement with Iran are fully implemented,” without directly addressing Tehran’s accusation that the US had breached the deal. The duelling statements came as Israeli airstrikes pounded southern Lebanon, which the Israel Defense Forces described as a response to Hezbollah launching more than 50 projectiles at its troops. Hezbollah, in turn, claimed it was repelling Israeli incursions and insisted on its “full right” to confront attacks.

The MoU, signed remotely by US President Donald Trump and Iranian President Masoud Pezeshkian on Wednesday night, laid out a framework for a comprehensive ceasefire including Lebanon, the reopening of the Strait of Hormuz, the lifting of the US naval blockade on Iranian ports, and the start of 60 days of negotiations aimed at a final settlement covering Iran’s nuclear programme, sanctions, and frozen assets. A formal meeting in Switzerland intended to advance those talks was cancelled on Friday amid the escalation in Lebanon. According to Iranian foreign ministry officials, a delegation is still to travel to Switzerland to “demand implementation of the other side’s commitments,” with Tehran insisting it is adhering to its own obligations and pressing Washington to compel Israel to halt its operations.

Viewed from regional capitals, the rapid unravelling of the ceasefire casts a shadow over the diplomatic process. Lebanese health authorities reported at least 23 fatalities from Israeli strikes on Saturday, bringing the overall toll since the war began on 2 March to more than 4,000 dead. Direct Lebanese–Israeli negotiations are scheduled to begin on 23 June. Analysts in London note that while the Iranian closure announcement may be intended to extract faster US enforcement of the ceasefire, the credible threat of blocking a chokepoint through which a fifth of global oil supplies passes adds enormous pressure on the incoming talks. The MoU’s economic incentives for Iran—potential release of $24 billion in frozen funds, a near-doubling of oil exports to 2.5 million barrels per day, and a proposed $300 billion reconstruction fund—are substantial but remain contingent on the restoration of security guarantees that the present flare-up directly challenges.