HMS Montrose
Royal Navy Type 23 frigate HMS Montrose performs a series of tight turns during exercise Marstrike 05 off the coast of Oman March 22, 2005. Picture taken March 22, 2005. Image Credit: Reuters

The stakes just rose for U.K. oil tankers passing through the Strait of Hormuz from the Arabian Gulf, after a Royal Navy frigate warned off three Iranian patrol boats that attempted to impede the passage of a BP Plc-owned vessel. But the problem remains a uniquely British one, for now.

What happened?

On Wednesday evening the HMS Montrose was escorting the oil tanker British Heritage through the chokepoint when its charge was approached by the Iranian ships. These reportedly attempted to force the Isle of Man-flagged tanker to change direction into Iranian waters. The Montrose maneuvered between the tanker and the patrol boats and ordered them to move away, which they did, according to a statement from the U.K. Ministry of Defence.

The Iranian foreign Minister Mohammad Javad Zarif and Iran's Islamic Revolutionary Guard Corp both rejected the British version of events. An IRGC statement issued Thursday said that "during the last 24 hours, there have been no encounters with foreign vessels, including the British ones," according to a report by Iran's semi-official Fars news agency. A U.S. aircraft was overhead and recorded footage of the episode, CNN reported, which could help settle the matter.

We will never know if the Iranian vessels would have tried to detain the British Heritage had the frigate not been shadowing it. Civilian ships transiting Hormuz have been detained by Iran before. In April 2015 the Maersk Tigris, a cargo ship flying the flag of the Marshall Islands, was seized by Iran in international waters. The incident related to a decade-old dispute over 10 cargo containers. The following month the U.S. Navy began accompanying U.S.-flagged ships through the chokepoint, though this stopped after about a week.

Its capture was one of three incidents involving merchant ships in and around Hormuz at that time. These coincided with rising tensions between hard-liners and more moderate factions in Iran's political system, and rising tensions over conflicts in the Mideast.

This time, Iran has made specific threats against British ships after U.K. special forces impounded the supertanker Grace 1 last week off Gibraltar. That vessel was alleged to be hauling around 2 million barrels of oil to Syria, which put it in breach of European Union sanctions. The crude originated in Iran, and its military has vowed to retaliate "" the country is one of the only suppliers left to President Bashar Al-Assad's government. The harassing of the British Heritage would seem to be the first evidence of that retaliation.

But this need not signal the start of wider problems for shipping through the Strait of Hormuz "" at least not yet. Iran still wants to find ways to export its own crude. Threatening international shipping through the chokepoint won't make that any easier, as its customers in China and elsewhere rely on passage through the strait for much of their oil imports.

There were eight Manx-flagged tankers in the Arabian Gulf in the past five days, according to tanker tracking data compiled by Bloomberg. Four of them are still there and will probably seek a military escort out of the region.

The HMS Montrose is the only British naval vessel in the region, according to the MOD, so it looks like it is going to be kept busy.