The US President Donald Trump's plan for a full naval blockade of the Strait of Hormuz threatens to deepen an already unprecedented crisis in one of the world's most important energy-producing regions.
The narrow waterway connecting the Persian Gulf to the wider world has become a flashpoint since the US and Israel began strikes on Iran six weeks ago.
A full blockade could halt the remaining flows and threaten economies far beyond the Middle East.
Transits have dwindled to single digits a day, down from about 135 in peacetime.
A US naval blockade along the lines of what was carried out off Venezuela from the end of last year could reduce that to zero, pressuring Iran but also cutting off a vital source of supply for Asian nations.
It would also likely doom an already fragile ceasefire agreed last week.
The US military separately issued a more narrow interpretation, setting a start of Monday at 10 a.m. Eastern Time for the blockade.
It applies to all vessels 'entering or departing Iranian ports and coastal areas,' adding that freedom of navigation through the corridor would not be impeded.
Seafarers are advised to monitor official broadcasts and contact US naval forces when in the Gulf of Oman and approaching the Strait of Hormuz, it added.
A blockade, if successfully enforced, would prove extremely painful for Iran, which relies heavily on its oil exports.
Over the past weeks, the country has benefited from higher prices and cargoes previously sold at a discount to global Brent found themselves at a premium earlier this month.
The higher selling price for each barrel is key for Iran, which has suffered major damage from US and Israeli airstrikes and will have to make significant investments to rebuild and prop up its ravaged economy.
