Iran’s Khatam al‑Anbiya Central Headquarters declared on June 20 that the Strait of Hormuz would again be closed to vessel traffic, describing the step as a “first response” to what it termed a US “breach of contract” and “continuous violation of the ceasefire by the Zionist regime in southern Lebanon.” The announcement came with a warning that further measures would follow if what Iran called “aggression” persisted. According to the Iranian military command, Washington had failed to implement the first clause of the memorandum of understanding (MoU) signed on June 17 – an immediate and permanent cessation of military operations on all fronts, including Lebanon. The declaration was made even as an Iranian negotiating team headed by Parliament Speaker Mohammad Bagher Qalibaf and Foreign Minister Abbas Araghchi departed for talks in Switzerland, and US Vice‑President JD Vance also began travelling to the same venue.

Viewed from Washington, the US response was swift and contradictory. Central Command insisted that “safe passage through this international waterway remained intact” with 55 merchant vessels transiting the strait on June 20, carrying more than 17 million barrels of oil. A spokesperson added that US forces were “present and vigilant” to ensure compliance with the MoU. President Donald Trump, posting on Truth Social, stated that no tolls would be charged during or after the 60‑day ceasefire period unless a final deal failed, in which case the United States might impose charges as “Guardian Angel” to the Middle East. Vance, in a Fox News interview, said he had seen no evidence the strait was actually closed and expressed confidence the ceasefire could be maintained. The White House signalled that it viewed the Iranian announcement as a pressure tactic rather than a definitive operational change.

The move by Tehran was triggered by renewed Israeli strikes in southern Lebanon that killed at least 16 people on June 20, hours after a ceasefire between Israel and Hezbollah had been announced. Israeli military officials, speaking on condition of anonymity, said Hezbollah had fired more than 50 projectiles at Israeli forces overnight, prompting retaliatory strikes on what it described as Hezbollah command centres and rocket sites. Hezbollah accused Israel of exploiting the truce to attempt an infiltration. From Beirut, Lebanese state media reported widespread air raids, and the health ministry noted mounting civilian casualties. According to regional sources, Iran has conditioned a reopening of the strait on a public Israeli commitment to a comprehensive ceasefire and withdrawal, while Israel – not a party to the MoU – maintains that its operations against Hezbollah are separate from the US‑Iran framework.

The closure announcement exposes the fragility of the MoU, which was intended to halt a conflict that began on February 28 and to open a 60‑day negotiation window on Iran’s nuclear programme and sanctions relief. Diplomats in Europe note that the agreement’s reliance on an external Lebanon ceasefire, without any enforcement mechanism over Israel, creates an acute vulnerability. With Pakistani and Qatari mediators already in Bürgenstock, Switzerland, technical‑level talks are scheduled for June 21. Iranian foreign ministry spokesman Esmail Baghaei warned that unless key commitments were honoured, “the memorandum of understanding as a whole will be jeopardised.” The immediate test will be whether the Swiss talks can stabilise the Lebanon truce and keep the broader diplomatic track alive.