Europe is studying proposals that may allow the charging of navigational fees in the strait of Hormuz so long as the tolls are not compulsory and have the support of the UN agency that regulates maritime transport.

Britain’s deputy prime minister, David Lammy, said the imposition of compulsory tolls would be disastrous. But some of his cabinet colleagues said they recognised that systems of payments for specific navigational services were permissible in many natural waterways, including the strait of Malacca and the Channel.

It comes as US officials demanded Iran make a public statement saying the strait of Hormuz is open and that ships using the vital corridor won’t be attacked any more. The US officials blamed power struggles inside Tehran for the difficulty in reaching and adhering to a deal.

Donald Trump reiterated on social media on Friday that he viewed the interim ceasefire deal as “over” but that the US would continue talks aimed at reaching a permanent end to the war.

Hours later, the US president later made fresh threats against Iran should it seek to assassinate him, saying “1000 missiles are locked and loaded and aimed at the Islamic Republic of Iran”.

A proposal for the strait that adapts the principles from the strait of Malacca has already been developed by Oman in conjunction with British lawyers. Muscat has now offered to send its legal experts to Tehran to explain the plan in detail.

Iranian state media reported on Friday that foreign Minister Abbas Araghchi would travel to Oman on Saturday for talks about the strait. The visit “will be focused on the strait of Hormuz and shipping safety” and is “a continuation of the consultations that we started with Oman over the past one or two months,” Iran’s official IRNA news agency quoted foreign ministry spokesman Esmaeil Baghaei as saying.

Oman controls most of the navigable waters in the strait and opposes a compulsory toll. Qatar’s foreign ministry spokesperson, Majed al-Ansari, said: “Giving the Iranians sovereignty over the strait in a way that contradicts international maritime law will be basically agreeing to be hostages to whatever radical element that wants to take over the strait at any time.”

But the scale of Oman’s alternative scheme may not match the ambitions of the Iranians, especially the Islamic Revolutionary Guard Corps. One diplomat said: “There are sections of the IRGC that say the US launched an unlawful attack on them in February, so why should they bother with the international law of the sea? Others want to cooperate. There is a division in Tehran.”

Iran is also under pressure from regional states to clarify its proposals, and whether the fees would in effect be compulsory.

The Iranian embassy in London said it was interested in proposals prepared independently by the Energy Policy Research Group.

The paper argued that a transparent service fee embedded within an inclusive regional order would incentivise all sides to cooperate, adding that the plan was not a crude toll imposed on ships just for passing through the strait.

Speaking at a meeting of the International Maritime Organization council on Thursday in London, the Oman delegate Khamis bin Mohammed Al Shamakhi said: “The right of transit passage through straits used for international navigation is guaranteed under international law and does not support the imposition of transit fees on vessels passing through the strait of Hormuz.”

However, he added that Oman saw value in exploring voluntary arrangements “relating to navigational support services that could further enhance maritime safety and security, protect the marine environment, reduce the risk of pollution and strengthen preparedness for maritime emergencies, including collisions and fires on board of ships and tankers”.

Behind these remarks is detailed work by Oman on the governorship of the strait of Malacca that links the Indian and Pacific oceans. A report to the IMO council on the cooperative mechanism for the straits of Malacca and Singapore said more than 120,000 vessels transit the strait annually.

The mechanism “has evolved into a structured and inclusive platform through which emerging risks, technological developments and environmental priorities in the straits may be collectively addressed”, the report added. Voluntary contributions, notably by Japan, keep the mechanism working.

At the London meeting, an alliance of some Gulf and European states pressed for a resolution condemning Iran for seeking to control the strait of Hormuz by attacking ships. The motion was not supported by Russia or China.

Russia said the confrontational motion completely ignored the root causes of the crisis, while China described the text as one-sided, and going beyond the IMO mandate.

The motion came after the US hit more than 150 targets mainly in the south of Iran this week, in an attempt to destroy the Iranians’ ability to harass shipping with drones, missiles and small boats. Iran has responded by hitting US bases in Kuwait and Bahrain.

One diplomat said there were two reasons for the renewed fighting. One concerned the disputed control of the strait during its reopening and the other was the long-term management of the waterway, including whether Malacca provided a model that would be acceptable to Iran.

Clause 5 of the memorandum of understanding (MoU), the ceasefire negotiation roadmap signed by Washington and Tehran last month, committed Iran to make its best efforts for the safe passage of commercial efforts through the strait with no charge for 60 days only.

Once technical and military obstacles had been removed, traffic of commercial vessels would be “instated” within 30 days.

The US has claimed the MoU did not mean ships could pass the strait only with Iran’s permission and only on routes specified by Tehran.

Separately, the memorandum committed Iran to hold talks on a long-term plan for the strait with Oman.

In a statement on Thursday, the IRGC navy claimed it had in fact met the MoU commitments, as it interpreted them.

The IMO secretary general, Arsenio Dominquez, believed he had won Iran’s agreement to the southern route that would allow thousands of stranded sailors to evacuate the strait, but, if there was an agreement, Tehran rescinded its approval, forcing the UN agency to suspend its plan.

The US nevertheless continued to encourage commercial traffic ships to use the southern route. US Central Command claims that since early May US forces “have helped facilitate the successful transit of more than 800 commercial vessels and 380m barrels of crude oil through the vital international trade corridor”.

The IRGC navy said: “We reiterate that foreigners have no role in this land or the strait of Hormuz.”

Diplomats are now examining whether Tehran is insisting all ships use the northern route close to Iran to clear the backlog of vessels or is simply requiring that all ships seek permission of the country and its Persian Gulf Strait Authority to use the southern route.