TEHRAN : Iran's Revolutionary Guards on Thursday warned against any crossings of the Strait of Hormuz without authorisation, saying vessels not complying "will be dealt with".

The future of the strait, a vital route for energy shipments that was locked down by Iran during the war, is a key sticking point in negotiations between Tehran and Washington.

Tehran has said it plans to impose what it calls maritime service fees, as opposed to tolls, while the United States argues it is an international waterway and therefore should not be charged.

"The only authorised route for passage through the Strait of Hormuz is the route announced by the Islamic Republic of Iran," said the Revolutionary Guards, the ideological arm of Iran's military.

Any crossing without authorisation is "unacceptable and extremely dangerous", they warned in a statement.

They also denounced what they said was a new route through the waterway announced by "certain authorities", without elaborating.

Hormuz is a narrow stretch of water between Iran and the Gulf countries through which roughly 20 percent of the world's crude oil and liquified natural gas normally transits.

At its narrowest it is only about 30 kilometres (18 miles) wide.

The only route currently authorised by Iran runs through a corridor that follows the country's coast.

A memorandum of understanding signed last week by Tehran and Washington to end their war stipulates that commercial ships may transit the strait free of charge for the next 60 days.

With Iran and the US in negotiations, it is unclear what arrangements will be in place after that period.

Iran and Oman, which also borders the strait, announced on Tuesday that they would study the "costs" to be charged for services related to administration of the strait.

But US Secretary of State Marco Rubio, visiting neighbouring Gulf countries this week, said Washington would not accept any tolls or fees.