Iran's Revolutionary Guard has captured British-flagged oil tanker Stena Impero and briefly detained a second vessel in the Strait of Hormuz.

Key points: British Foreign Secretary Jeremy Hunt said Iran's seizure of the Stena Impero was unacceptable

British Foreign Secretary Jeremy Hunt said Iran's seizure of the Stena Impero was unacceptable Iranian state TV claims the tanker turned off its tracker and ignored warnings from Revolutionary Guards

Iranian state TV claims the tanker turned off its tracker and ignored warnings from Revolutionary Guards The incident comes after Iran and the US emphatically disagreed over Washington's claim that an American warship downed an Iranian drone near the Persian Gulf

The IRGC seized tanker Stena Impero at the request of the maritime authorities in the Iranian province of Hormozgan for "not following international maritime regulations," state television cited an IRGC statement as saying.

The tanker was taken to a coastal area and turned over to the authorities to take the necessary legal steps, state television said.

Iran's state television IRNA quoted a military source who claimed the Stena Impero had turned off its tracker and ignored warnings from the Revolutionary Guards.

On Saturday, Iran's Fars news agency reported that the UK tanker had earlier collided with an Iranian fishing boat and ignored its distress call, citing an official.

All 23 crew on the tanker were at Bandar Abbas port and would remain on the vessel until the end of an investigation, Fars quoted the official as saying.

The Mesdar tanker abruptly turned north towards Iran, before returning to its voyage west through the Strait of Hormuz. ( Marinetraffic.com )

The same source claimed the tanker's violations also included sailing in the wrong direction in a shipping lane and ignoring directives.

A statement from Stena Bulk, which owns the seized tanker, said it was unable to make contact with the ship after it was approached by unidentified vessels and a helicopter in international waters. The ship has 23 crew aboard.

Meanwhile, tracking data showed a second oil tanker, the British-operated, Liberian-flagged Mesdar, had turned west, away from Iran, after earlier turning sharply north in the strait, according to Refinitiv tracking data.

The semi-official Tasnim news agency reported Iran had not in fact captured the Mesdar, quoting regional military sources.

"Despite reports, the ship has not been seized … and was allowed to continue its course after being warned about safety issues by Iranian forces," the report said.

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The Mesdar's northerly turn had taken place at about 2:00am (AEST), about 40 minutes after a similar course shift by the Stena Impero tanker.

There was no immediate word from the IRGC 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.

British Foreign Secretary Jeremy Hunt said Iran's seizure of the Stena Impero was unacceptable and, in comments reported by Sky News, that Iran will be the biggest loser if freedom of navigation is restricted.

"I'm extremely concerned by the seizure of two naval vessels by Iranian authorities in the Strait of Hormuz," Mr Hunt said earlier, before concerns abated over the Mesdar.

"These seizures are unacceptable. It is essential that freedom of navigation is maintained and that all ships can move safely and freely in the region," he said.

Mr Hunt said he has spoken to US Secretary of State Mike Pompeo about the seizures

Watch Duration: 25 seconds 25 s Watch Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. UK Foreign Secretary warns of 'serious consequences' after British oil tankers seized

'This incident represents an escalation'

Further protection must be provided for merchant vessels, the head of the UK Chamber of Shipping said.

"We condemn unreservedly the capture of Stena Impero as she transited the Strait of Hormuz earlier today," the Chamber's chief executive, Bob Sanguinetti, said in a statement.

"This incident represents an escalation. Whilst we call for measured response, it is also clear that further protection for merchant vessels must be forthcoming to ensure enhanced security to guarantee free flow of trade in the region."

The Mesdar's tracking showed it turning sharply north towards the Iran coast after sailing in a westerly direction through the strait. ( A MacKinnon via marinetraffic.com )

The Australian Government said it was deeply concerned by heightened tensions in the Gulf.

"We strongly condemn the recent attacks on shipping in the Gulf of Oman. Australia relies on freedom of navigation and the uninterrupted passage of maritime trade," a spokesman from the Department of Defence said in a statement.

"As a matter of course, Australia consults closely with our allies and partners on the situation in the Middle East.

"We are following the situation closely, including through our embassies in the region."

Iran continues to deny its drone was shot down

US President Donald Trump said on Saturday (AEST) he would talk to the United Kingdom after the seizure of the tankers.

"This only goes to show what I'm saying about Iran: Trouble, nothing but trouble," he said.

The US military has said that unarmed surveillance aircraft were in international airspace, monitoring the Strait of Hormuz and had been in contact with US ships in the area as Britain said Iran seized two oil tankers.

"We have patrol aircraft operating in international airspace monitoring the situation within the Strait of Hormuz," said Lieutenant Colonel Earl Brown, a US Central Command spokesman.

The incident came as Iran and the United States emphatically disagreed on Friday over Washington's claim that a US warship downed an Iranian drone near the Persian Gulf.

American officials said they used electronic jamming to bring down the unmanned aircraft, while Iran said it simply did not happen.

The United States is aware of reports that Iran seized the tanker and it will work with its allies and partners to defend against Iran, a White House National Security Council spokesman said.

"We are aware of reports that Iranian forces seized a British oil tanker," spokesman Garrett Marquis said.

"The US will continue to work with our allies and partners to defend our security and interests against Iran's malign behaviour."

AP/Reuters