Oman has been caught in geopolitical crossfire after Iran said it was coordinating with the Gulf state over the future management of the strait of Hormuz, including Tehran’s plans to impose fees on commercial shipping.

The Omani exclave of Musandam lies to the south of the contested waterway, which normally carries a fifth of the world’s seaborne oil traffic but has been blockaded for 10 weeks since the US-Israeli attack on Iran in February.

The US has said repeatedly there can be no permanent solution to the blockade that involves the payment of a toll to Iran, and claims that Oman holds a similar view.

Speaking in India on Friday, the Iranian foreign minister, Abbas Araghchi, defined the strait of Hormuz as an exclusively Omani-Iranian waterway. “The strait is located in the territorial waters of Iran and Oman,” he said. “There is no international waters in between.”

Araghchi added that Iran was coordinating with Oman about the future management of the strait. Oman has so far been silent about Iran’s plans to charge a fee and to demand details on the nationality of all ships passing through the waterway.

Western diplomats say Iranian proposals for the future permanent management of the strait are unlawful since they impose tolls on commercial shipping and would give Iran an arbitrary right to select the ships that are allowed passage, possibly based on the nationality of ownership.

A requirement that every ship set up a rial account to pay for services would also probably fall foul of UN sanctions prohibiting money being sent to the Islamic Revolutionary Guard Corps (IRGC).

A rival plan based on freedom of navigation being prepared by France and the UK has also been put to Oman, and has the support of most Gulf states.

British officials including the Foreign Office’s political director, Lord Llewellyn, have recently been in Muscat, as has the secretary general of the International Maritime Organization, Arsenio Dominguez.

The legal rights of coastal states to impose tolls lies at the heart of the deadlock on how to reopen the strait and whether Iran’s proposed move, by restricting freedom of navigation, is illegal and sets a precedent for other similar waterways.

Iran became a signatory to the UN convention on the law of the sea (Unclos) in 1982, very soon after the 1979 revolution, but never ratified the treaty.

This means that from Iran’s perspective it is not bound by the treaty’s transit passage rules that underpin freedom of navigation, but instead by customary international law, including a more restrictive right of innocent passage.

Iran claims that even if it is bound by Unclos, the enhanced right of transit passage to ships of nations is conditional and passage specifically can be restricted in the event of any threat or use of force against “sovereignty, territorial integrity or political independence of the coastal states”.

Tehran said at the outset of the conflict that the southern shore of the strait including the United Arab Emirates was used by the the US to arm American bases to attack Iran.

Iran hopes the Persian Gulf Strait Authority (PGSA), a government agency it established on 5 May, will become a profitable revenue-generating stream.

A grey area is whether Iran can only charge fees in return for providing services to ships, or whether in reality it would be compulsory to use those services, so in effect turning the service fee into a toll.

The PGSA says ships now have to register by email with its office to receive routing information and permission to pass the strait.

Payment would be required in Iran’s national currency. The fee is being set at roughly a dollar a barrel.

Donald Trump at his summit in Beijing claimed that China – which imports almost 45% of Iran’s oil production through the strait – agreed with the US that there could be no tolls and no restrictions. The US secretary of state, Marco Rubio, has also said China does not favour tolls.

The Chinese foreign ministry said it simply wanted the blockades to end and that the cause of the closure was the US-Israel war on Iran.

However, the IRGC briefed on Thursday that after talks with China’s ambassador to Iran, a large group of Chinese oil tankers were being let through the strait by Tehran and that these ships had agreed to be subject to the Iranian regime. The wording did not reveal whether a fee was paid by China.

At the time the US imposed its blockade of Iranian ports, as a countermeasure to Iran’s effective closure of the strait, Trump said: “No one who pays an illegal toll will have safe passage on the high seas.”

That implied the US navy might feel entitled to block Chinese oil tankers if they paid a toll to Iran. However, it would be hard to gain evidence at the relevant moment whether a toll, or fee had been paid.