British armed forces supported the US seizure of a Russian-flagged oil tanker in the North Atlantic on Wednesday, the Ministry of Defence has said.

The Marinera, a Venezuelan-linked tanker previously known as the Bella 1, was stopped by the US Coast Guard, as it travelled northwards through waters between Iceland and Scotland.

The MoD said the US asked the UK for assistance, and RAF surveillance aircraft and a Royal Navy support ship, the RFA Tideforce, took part in the operation.

Defence Secretary John Healey said the action was “in full compliance with international law”, adding the UK “will not stand by as malign activity increases on the high seas”.

In a statement to MPs on Tuesday explaining why the UK joined the operation, Healey said the government was “stepping up activity against shadow vessels”.

In a post on X, the US military’s European Command thanked the MoD for its “unwavering support” during the operation.

A Downing Street spokesperson said Prime Minister Sir Keir Starmer discussed the joint operation, as well as recent talks on Ukraine and the US operation in Venezuela, in a phone call with President Trump on Wednesday evening.

In a brief statement, the spokesperson said the PM “set out his position on Greenland” during the call – the first to take place between the two leaders since the US seizure of Venezuelan President Nicolás Maduro at the weekend.

On Tuesday Sir Keir told the BBC only Greenland and Denmark should decide the future of the territory, after Trump repeated his threats to annex it.

The Marinera is accused of being part of shadow fleet carrying oil for Venezuela, Russia and Iran, breaking US sanctions.

The US Coast Guard had a warrant to seize the ship, and had tried to board the tanker last month in the Caribbean. But the tanker dramatically changed course, changed its name, and re-registered as a Russian vessel, and for two weeks the US has been pursuing it.

Healey said: “By assisting our US allies in taking this ship off the seas, we are protecting the British people and our nation.”

He said the ship had a long history of “nefarious activity” linked to Russia and Iran, and that Iran was a “persistent danger” to the UK.

The vessel, which has been sanctioned by the US since 2024, had moved 7.3 million barrels of oil on behalf of Iran, Healey said, “the proceeds of which have been used to finance terrorism, threat and instability across the world”.

He also said the shadow fleet was “vital to Putin’s ongoing illegal invasion and war in Ukraine”.

Shadow fleets, or ghost fleets, are becoming increasingly common, with Venezuela, Iran and Russia all accused of using them to avoid sanctions on oil.

Financial intelligence firm S&P Global estimates that one in five oil tankers worldwide are used to smuggle oil from sanctioned countries. The ships are often ageing tankers and use false flags to try and avoid detection.

Healey praised the “courage” of the US forces in the operation, and said no UK personnel took part in the boarding of the ship.

The seizure of the Marinera came as the US said it had seized the M/T Sophia, another tanker with connections to Venezuela, in the Caribbean – and days after a military operation in Caracas to remove President Nicolás Maduro from power.

In a statement, the MoD said the Marinera was also assessed to have been involved in illegal activity linked to Lebanon-based militant group Hezbollah.

It said: “The ship, initially flying a false flag, turned off its transponders while at sea and sought to reflag while being pursued.”

Moscow has denounced the seizure of the tanker sailing under its flag, and demanded that the US treat Russians on board properly and permits them to return to Russia quickly.

The transport ministry said it had given the vessel “temporary permission” to use the Russian flag, adding that no state had the right to use force against vessels properly registered in other states’ jurisdictions.

Reports had suggested that Russia had dispatched a submarine to safeguard the vessel, but it appears that US forces were able to board the tanker without facing any resistance.

The seizure of the two tankers was announced by the US military in separate posts on social media on Wednesday.

Experts told BBC Verify that under UN international maritime law, a stateless vessel can be boarded by authorities.

Venezuela’s leadership is co-operating with the US on the second tanker seized in the Caribbean, Secretary of State Marco Rubio said.

Homeland Security Secretary Kristi Noem said that “in two predawn operations today, the Coast Guard conducted back-to-back meticulously co-ordinated boarding of two ‘ghost fleet’ tanker ships”.

She said both seized vessels “were either last docked in Venezuela or en route to it”.