A fifth of the world's oil and liquefied natural gas passed through the strait in peacetime -- before the United States and Israel launched strikes on Iran on February 28, prompting it to retaliate across the region.

- How are ships crossing? -

Maritime trackers indicate that a handful of vessels are still crossing daily through the strait, which is guarded by Iran's Revolutionary Guard Corps (IRGC) -- most of them leaving the Gulf.

Some have taken a new Iranian-approved route through its waters, dubbed the "Tehran Toll Booth" by leading shipping journal Lloyd's List.

At least one vetted vessel paid $2 million to use the corridor around Larak Island just off Iran's coast, it reported.

Lloyd's List cited sources as saying that governments from countries including India, Pakistan and China discussed vessel transits directly with Tehran, with the IRGC establishing a registration system of "approved ships".

On Tuesday, IRGC naval commander Alireza Tangsiri said on X the force had "turned back" a container ship "due to failure to comply with legal protocols and lack of permission to pass through".

Ship tracker MarineTraffic showed that the vessel, the Selen, was bound for Pakistan.

- What vessels have used the corridor? -

Lloyd's List reported Monday that it had tracked more than 20 ships using the emerging route, most of them Greek-owned but others Indian-, Pakistani-owned or linked to China.

In a sign that commodity vessels were gaining approval to use the route, since the start of last week four that crossed the Strait of Hormuz kept their AIS transponders on while using the corridor, while at least five more that crossed sent signals in close proximity to the passage, an AFP analysis of Kpler data showed.

Over half of those crossing since Sunday specified the nationality of their owner, crew or cargo using their AIS signals, it showed.

By comparison, from March 1 to 21 only about 10 percent of ships navigating the strait did so.

"Vessels are using AIS messages to signal political neutrality or distance from Western, Israeli, or otherwise sensitive affiliations," Kpler trade risk analyst Ana Subasic told AFP.

But she noted they are "self-declared, easily altered, and do not change a vessel's legal identity" and therefore should not be considered "a reliable security solution".

- What has Iran said publicly? -

In a statement dated Sunday and released late Tuesday via the International Maritime Organization, Tehran reiterated its stance that the strait was open, but only to some.

It said "non-hostile vessels" and those not "belonging to or associated with" Israel, the US and other "aggressors" could cross "subject to compliance with... the realities arising from the ongoing conflict".

The foreign ministry statement added that ships linked to qualifying countries could "benefit from safe passage" but "in coordination with the competent Iranian authorities".

- What does international law say? -

The United Nations Convention on the Law of the Sea -- which was adopted in 1982 and came into force in 1994 -- guarantees a right to navigate through straits such as Hormuz.

"All ships and aircraft enjoy the right of transit passage... for the purpose of continuous and expeditious transit of the strait," it states.

Although Iran has never ratified the convention, "the transit passage regime is widely regarded as part of customary international law," Marco Roscini, international law professor at Westminster Law School, told AFP.

- Is Iran acting illegally? -

Charging fees to cross the strait would "lack a valid legal basis", Roscini said.

Shipowners risk violating the law and Western sanctions if they pay a toll.

Transit passage rights through straits remain applicable during armed conflicts, and suspending commercial navigation through Hormuz "cannot be lawful", he added.

In exceptional circumstances, including the right of self-defence, states can "impose temporary restrictions on navigation if this is necessary and proportionate".

In this case, Roscini said, "I am not clear whether Iran has explicitly invoked self-defence".

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