The International Maritime Organization's Secretary General Arsenio Dominguez addressed a news conference as access to the strait remained blocked six weeks after the war erupted with US and Israeli strikes against Iran.

The United States had threatened to begin a blockade on Monday of Iranian ports in and around the strait, which Tehran's forces have been controlling access to since after the war broke out on February 28.

"In accordance to international law, no countries have the right to prohibit the right of innocent passage or the freedom of navigation through international straits that are used for international transit," Dominguez said.

Iranian authorities have been allowing a trickle of vetted vessels to pass the strait through a route close to their coast and in some cases have reportedly levied a payment to let vessels through.

"This principle of introducing a toll on an international strait for international navigation is against the international law of the sea and the customary law," Dominguez said.

"It will create a very dangerous precedent."

The US vow to blockade Iranian ports meanwhile "doesn't make it any easier", he added.

"De-escalation is what is going to start helping us to address the crisis and to bring shipping back to the way that we used to operate."

He predicted that the extra impact of a US blockade on shipping would be negligible, however.

"With the very few number of ships that have managed to transit, an additional blockade is not going to exacerbate the situation in a level that it could be perceived."