Two British oil tankers have been seized by Iran and is heading towards a Revolutionary Guard base, in a major escalation of tensions along one of the world's most vital oil shipping routes.

The Stena Impero had been en route to Saudi Arabia but made an abrupt change of course and began moving towards the Iranian island of Qeshm, according to data relayed by maritime tracking services.

The ship "went dark", meaning its identification system was turned off, at 3.30am (Saturday NZT) and nothing has been heard from her or her 23 crew since.

The second vessel, the Mesdar, which is Liberian flagged but owned by British company Norbulk Shipping, was ordered by Iran to turn north around 45 minutes later. It was later released after being boarded and inspected by Iranian troops.

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The Stena Impero was passing through the bottle neck into the Persian Gulf when it turned dramatically to the north.

The vessel was surrounded by heavily-armed small craft and a helicopter and ordered to turn north.

The Mesdar turned sharply north towards Iran's coast around the same time the Stena Impero was seized.

The turn took place at about 4am (NZT) the data showed, about 40 minutes after a similar course shift by the Stena Impero tanker that Iran's Revolutionary Guards said they had captured.

"These seizures are unacceptable," said British Foreign Secretary Jeremy Hunt said as he prepared to enter an emergency government meeting in the UK Friday night. "It is essential that freedom of navigation is maintained and that all ships can move safely and freely in the region."

This undated photo provided by Iranian state TV shows a Panamanian-flagged oil tanker surrounded by Iranian Revolutionary Guard vessels. Photo / AP
This undated photo provided by Iranian state TV shows a Panamanian-flagged oil tanker surrounded by Iranian Revolutionary Guard vessels. Photo / AP

There was no immediate word from the Guards about the second tanker or from the operator of the second tanker on what had prompted the change in direction along the vital international oil shipping route.

The Iran Revolutionary Guard's website, sepahnews.com, says Stena Impero was seized by Iranian forces for "non-compliance with international maritime laws and regulations" and has transferred the vessel to an Iranian port.

The report did not elaborate what port it was transferred to.

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The operator of the oil tanker says it is unable to contact the ship after it was approached by unidentified vessels and a helicopter.

Northern Marine Management and shipping firm Stena Bulk say in a statement that the vessel was in international waters and is now heading toward Iran.

The vessel had left Fujairah in Dubai and moved into the Strait of Hormuz when it was intercepted.

The British government says it is urgently seeking information after reports Iran has seized the British-flagged ship.

President Donald Trump says the USS Boxer destroyed an Iranian drone in the Strait of Hormuz amid heightened tensions between the two countries. Photo / AP
President Donald Trump says the USS Boxer destroyed an Iranian drone in the Strait of Hormuz amid heightened tensions between the two countries. Photo / AP

British authorities seized the Iranian Grace 1 supertanker off the coast of Gibraltar on July 4, on suspicion it was carrying crude to Syria in violation of European Union sanctions.

The fate of the tanker has been at the centre of escalating tensions between the UK and Iran and seen as a pawn in the standoff between the Islamic Republic and the West.

The latest incidents will only increase fears for security along the Strait of Hormuz, through which almost one-fifth of the world's oil passes.

Oil prices rose in reaction to the news.

After one of the worst performing weeks since May, oil started the day firmer but slipped as the US and Iran continued to trade brickbats. The later rise initially still left it well down on the previous week. Oil was down more than 8pc this week overall when markets in London closed.

Iran has threatened to close the Strait if it cannot export its oil. The Trump administration is trying to block Iran's exports as a way to pressure it to renegotiate the landmark 2015 nuclear deal it abandoned last year.

The UK, which is understood to have seized the Grace 1 after a request from the US, is trying - alongside the EU - to keep the accord alive, believing it is the best chance to stop Tehran acquiring a nuclear weapon.

Revolutionary Guards have been threatening retaliation for its impounding and the move would likely have aggravated an already-tense situation.

Tensions have been building for weeks in the Persian Gulf.

On 10 July, a British warship, the HMS Montrose, intervened to drive three Iranian military vessels that were attempting to divert the British Heritage.

Iran seized a Panama-flagged ship on Sunday, it alleges, for "smuggling oil to foreign countries". However, mystery has surrounded the capture as no country has come forward to claim the ship or its cargo.

The vessel, however, was only carrying a very small amount and it had been thought Iran had seized it as merely a show of strength.

The US then on Thursday claimed to have downed an Iranian drone that had been flying too close to one of its navy ships.

The USS Boxer, an amphibious assault craft, destroyed the drone after it came within 1,000 yards in the Strait of Hormuz, at the entrance to the Gulf

However, Iran denied the claims and released footage on state TV to proof it was still in possession of the drone.

At the White House on Friday, President Donald Trump said flatly of the Iranian drone: "We shot it down." But Pentagon and other officials have said repeatedly that the USS Boxer, a Navy ship in the Strait of Hormuz, actually jammed the drone's signal, causing it to crash, and did not fire a missile. The officials spoke on condition of anonymity to discuss sensitive technology.

Trump's national security adviser, John Bolton, said, "There is no question this was an Iranian drone, and the USS Boxer took it out as the president announced yesterday because it posed a threat to the ship and its crew. It's entirely the right thing to do."

In Tehran, the Iranian military said all its drones had returned safely to their bases and denied there was any confrontation with the USS Boxer, an amphibious assault ship.

- AP, Daily Telegraph and Daily Mail