The Stena Impero had turned off its tracker and ignored warnings, Press TV also reported, citing Iranian military sources.
US defense officials confirmed that the tanker had been seized.
Later in the day, US officials told CNN that another ship — the Liberian flagged MV Mesdar tanker — had also been seized by Iran on Friday.
The order of seizure of the two ships isn’t clear at this time.
In a statement, the Stena Impero’s owner Stena Bulk and operator Northern Marine Management said that the Stena Impero had been “approached by unidentified small crafts and a helicopter during transit of the Strait of Hormuz while the vessel was in international waters” at approximately 4 pm local time (12 pm ET).
There are 23 seafarers aboard the ship, they added, saying that they are currently unable to make contact with the vessel, which is now heading north towards Iran.
The US military was monitoring the transit of a US commercial cargo ship through the Strait of Hormuz using armed aircraft overhead on Friday evening, according to a US defense official with direct knowledge of the situation.
The official would not say the location or provide any other details, however, that transit typically takes six to eight hours.
In a statement on Friday evening, UK Foreign Secretary Jeremy Hunt said that he was “extremely concerned by the seizure” of the two vessels. He added that the British Ambassador in Tehran is in contact with the Iranian Ministry of Foreign Affairs to “resolve the situation.”
“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.
Hunt added that the ships’ crews were comprised of a number of nationalities but that no British citizens were on board either ship.
A UK government official told CNN on Friday that the UK has convened a COBRA meeting — an emergency response committee that meets when there’s a domestic or international crisis — on the seized tanker.
On Friday afternoon, US President Donald Trump praised his country’s “close alliance” with the UK, saying that it will work with Britain on the incident.
He then claimed that the episode proved his warnings about Iran correct. “This only goes to show what I’m saying about Iran. Trouble. Nothing but trouble,” he said. “It goes to show you I was right about Iran,” he said.
Richard Meade, the managing editor of shipping industry publication Lloyds List, described the significance of the Stena’s seizure as “huge” for the region.
An escalating series of maritime episodes
On Thursday, the US Navy destroyed an Iranian drone using electronic jamming, a US defense official told CNN. The crew of the USS Boxer took defensive action against the Iranian unmanned aerial vehicle after it came close to the US naval ship, the official said.
Chief Pentagon Spokesperson Jonathan Hoffman said then that the USS Boxer had been in international waters where it was “conducting a planned inbound transit of the Strait of Hormuz.”
US President Donald Trump said the Iranian drone was “threatening the safety of the ship and the ship’s crew” and was “immediately destroyed.”
However, Iranian officials say none of their drones have been downed.
In another incident last week, armed Iranian boats tried unsuccessfully to impede the passage of a British oil tanker in the Persian Gulf, according to two US officials with direct knowledge of the incident.
And in June, tensions between the US and Iran escalated into a military standoff after an American drone was shot down by Iran over the Strait of Hormuz, one of the world’s most vital shipping routes.
This is a developing story….
CNN’s Max Ramsay contributed to this report.