A Royal Navy warship has escorted a British oil tanker out of the Persian Gulf as three Iranian vessels attempted to intercept it, according to the UK Ministry of Defence. The claim has been strongly denied by the Iranians.

The tanker ‘British Heritage’ had been in Saudi waters for several days due to concerns it may be seized upon entering Iranian waters. Tehran has threatened Britain with retaliation over its seizure of an Iranian oil-tanker off Gibraltar on July 4, which it claims as proof that the UK is participating in the US-imposed blockade of its oil exports. The EU, of which the UK remains a member, continues to oppose the sanctions. Iran’s Foreign Minister Mohammad Javad Zarif described the UK’s actions as ‘piracy.’

It was against this background of flaring tensions that US Chief of Staff General Joseph Dunford outlined plans on Tuesday for an American-led naval coalition to oversee maritime traffic in the Persian Gulf and Red Sea. “We’re engaging now with a number of countries to see if we can put together a coalition that would ensure freedom of navigation both in the Straits of Hormuz and the Bab al-Mandab… I think probably over the next couple of weeks we’ll identify which nations have the political will to support that initiative and then we’ll work directly with the militaries to identify the specific capabilities that’ll support that,” General Dunford said in his press conference explaining the initiative.

According to General Dunford, the US would occupy a leadership and surveillance role while most of the patrols would be carried out by allied states in support of their own commercial vessels.

The original announcement of the coalition had been overshadowed in June by Iran’s downing of a US surveillance drone in what it claims were its territorial waters, and a series of attacks on oil tankers in the Gulf of Oman which the US and its allies have blamed on Iran. Tehran has long relied on its control of the Straits of Hormuz to put pressure on the United States and its allies. In response to the US campaign of ‘maximum pressure,’ it has repeatedly warned that other regional states’ energy exports will be prevented if its own shipments are stopped.

Last week a proposal was raised in the Iranian Parliament, calling for the imposition of a toll on ships using the Strait. The move has so far been considered symbolic, but signals the degree to which control of international shipping routes is becoming a factor in the crisis between Washington and Tehran. The straits of Hormuz and Bab al-Mandeb each see the passage of roughly 19 million and four million barrels of oil a day respectively, making them among the most strategically sensitive shipping lanes in the world.

The Bab al-Mandab, which separates Yemen from the Horn of Africa, controls the most direct route between Asia, Africa and Europe, and by extension, affects traffic through the Egyptian-controlled Suez Canal.

The Houthi-movement in Yemen, fighting the US-backed Arab Coalition, has on occasion targeted Saudi and Emirati warships off the coast and frequently threatened to disrupt traffic in the Red Sea in retaliation for the Saudi blockade on the country.

So far, public international support for the US initiative has been sparse. Boris Johnson, the lead candidate for the ruling Conservative Party has suggested a British government under his leadership would actively participate in the US sanctions regime. His rival, Jeremy Hunt, has also announced to the House of Commons he would ‘consider’ future US requests for military assistance in the region. A similar undertaking has been given by the Australian Prime Minister Scott Morrison.

Such a coalition would struggle to gain United Nations approval, as the US sanctions on Iranian energy exports are in violation of the Iran nuclear deal or JCPOA which continues to be supported by the remaining signatories including China, Russia and the European Union. The French government has ramped up negotiations with Iran in the last week to find ways of keeping the accord alive.