This despite Iran blockading the waterway since the start of the Middle East war on February 28, preventing a large share of Gulf countries' hydrocarbon exports, in turn harming the global economy.

"The last week we saw ships flagged with Singapore, UAE, South Korea, and also a Norway flagged-vessel going through the Gulf, specifically exiting," said Bridget Diakun, an analyst at Lloyd's List Intelligence.

Crossings by very large crude carriers not tied to Iran appear in particular to be picking up again.

Of the 27 recorded by analytics firm Kpler since the start of the conflict, more than half took place in May.

Five of these huge oil tankers meanwhile left the Gulf through the strait between May 20 and May 26.

Three of them -- the Eagle Veracruz sailing under the Singaporean flag, as well as the Eagle Verona and the Yuan Gui Yang each flying the Chinese flag -- have declared China as their destination.

Another ship, the Universal Winner, is heading to South Korea, whose flag it flies, while the Nissos Keros, flying the flag of the Marshall Islands, is headed for India.

China, South Korea, India and Japan have coordinated with the Iranian government to ensure safe passage, according to Lloyd's List Intelligence.

On May 18, Iran formalised the creation of the Persian Gulf Strait Authority to manage navigation in the Strait of Hormuz and collect transit fees.

The United States strongly opposes the new body, with the US Treasury on Wednesday announcing sanctions against the PGSA, while threatening similar action against anyone paying the fees.

Such retaliation risks reducing the number of ships exiting the strait in coordination with Iran.

Adding to the uncertainty, Iranian forces have fired at four ships attempting to cross the Strait of Hormuz without authorisation, state broadcaster IRIB reported Thursday.

It came as the United States and Iran accused each other of violating an ongoing truce following an exchange of fire, three months after the Middle East war began with a wave of US-Israeli strikes on the Islamic republic.