Iran's Islamic Revolutionary Guards Corps (IRGC) says it has "confiscated" a British tanker, the Stena Impero, in the Strait of Hormuz for what it alleges was a failure "to respect international maritime rules."
According to the operator Northern Marine the vessel was approached by helicopters and other ships.
Early on Saturday Iran announced that the tanker was involved in an accident with an Iranian fishing boat and all its 23 crew members are on the vessel anchored at Bandar Abbas. The Iranian statement added that the tanker and its crew will remain there until "the investigation" is concluded.
Iran had threatened seizing a British tanker since Gibraltar detained an Iranian super-tanker in early July for carrying oil to Syria in violation of EU sanctions.
Recently Islamic Revolution Guard Corps speedboats tried to seize another British tanker but were driven away by a British warship.
British Foreign Secretary Jeremy Hunt said his country is focusing on diplomacy and not "military options" following Iran's seizure of a British oil tanker in the strategic Strait of Hormuz.
"We will respond in a way that is considered but robust and we are absolutely clear that if this situation is not resolved quickly, there will be serious consequences," Hunt told reporters late on July 19.
He added, though, that London was "not looking at military options -- we are looking at a diplomatic way to resolve the situation."
The incident is bound to increase tensions in an already volatile situation.
A White House National Security Council spokesman said on Friday The U.S. Is aware of the incident and it will work with its allies to defend against Iran.
"We are aware of reports that Iranian forces seized a British oil tanker," spokesman Garrett Marquis said.
"The U.S. will continue to work with our allies and partners to defend our security and interests against Iran’s malign behavior," he added.
The U.S. announced on Thursday it had downed an Iranian drone approaching a warship.
A second oil tanker, the British-operated, Liberian-flagged Mesdar, also turned sharply north toward Iran's coast on Friday afternoon after passing westward through the Strait of Hormuz into the Gulf. Fears emerged that Iran had seized two tankers.
But an Iranian news agency later said the Mesdar had not been seized but had been issued a warning, and tracking data showed it had turned away from the Iranian coast.
Refinitiv data showed the Stena Impero was a British-flagged vessel owned by Stena Bulk. It showed the vessel's destination as the Saudi port of Jubail on the Gulf.
The map tracking the ship's course showed it veering off course with a sharp turn north at about 1517 GMT on Friday and heading towards the Iranian coast.
The United States accused Tehran of "escalatory violence" and President Donald Trump spoke with French President Emmanuel Macron about the perceived Iranian threat and said he would "talk to the U.K." and "be working with the U.K." in light of the latest developments.
Trump added: "This only goes to show what I'm saying about Iran: Trouble, nothing but trouble."
The fresh incidents follow a month of naval confrontations, seizures, and reported shoot-downs of drones involving Iranian and Western vessels, along with accusations and counteraccusations of what happened and who is to blame.