US President Donald Trump on July 13 announced the reinstatement of a blockade on Iranian shipping in the Strait of Hormuz and a 20% levy on transiting cargo. The announcement was made in a post on Trump's Truth Social platform.

The measure, which Trump styled "the Iranian blockade", would bar Iranian vessels and ships serving Iranian customers from entering or leaving the strait, while all other countries would retain what he called fair and open use of the waterway.

"The Hormuz Strait is OPEN, and will remain OPEN, with or without Iran," he wrote.

Trump said the US would henceforth be known as the "Guardian of the Hormuz Strait" and, "as a matter of FAIRNESS", would be reimbursed at a rate of 20% on all cargo shipped through it to cover the costs of providing security in what he described as a highly volatile part of the world. "The process and formation will begin immediately," he added.

The post gave no detail on how the levy would be assessed or collected, its legal basis, or whether Gulf littoral states and other trading partners had been consulted.

The strait, which connects the Persian Gulf to the Oman Sea between Iran and Oman, carries roughly a fifth of global oil consumption and around a third of seaborne liquefied natural gas trade, the bulk of it Qatari. Its main users include Saudi Arabia, the UAE, Kuwait, Qatar and Iraq, with the largest volumes bound for Asian buyers.

The reference to Iran's "customers" suggests the interdiction would extend beyond Iranian-flagged vessels to tankers lifting Iranian crude, the overwhelming majority of which is shipped to China, raising the prospect of confrontation with third-country shipping.

The announcement marks a sharp escalation of economic pressure at a fragile point in the US-Iran conflict, which began in late February. A blockade on Iranian shipping was imposed earlier in the war before being eased, and the durability of the current ceasefire remains uncertain following the collapse of the Islamabad memorandum of understanding signed in June.

There was no immediate response from Tehran, nor from Arab Gulf governments or major shipping and insurance markets, where war-risk premiums for Hormuz transits have fluctuated sharply since the outbreak of hostilities.

A 20% ad valorem charge on cargo would represent an unprecedented toll on one of the world's most strategically important chokepoints, through which there is a right of transit passage under the UN Convention on the Law of the Sea.