The tanker "Impero Stena" was seized Friday by IRGC forces for "non-compliance with international maritime laws and regulations" and has transferred the vessel to an Iranian port, said the Guards sepahnews.com website.
The operator of the tanker said it is unable to contact the ship after it was approached by unidentified vessels and a helicopter in the Strait of Hormuz.
Northern Marine Management and shipping firm Stena Bulk said in a statement that the vessel was in international waters and is now heading toward Iran. The ship has 23 crew aboard.
The British government said it is urgently seeking information after reports Iran has seized the British-flagged ship.
The White House National Security Council said the United States “will continue to work with allies and partners to defend against Iran’s malign behavior”.
Relations between Iran and the West have been increasingly strained since British naval forces seized an Iranian tanker in Gibraltar on July 4 on suspicion of smuggling oil to Syria in breach of European Union sanctions.
Iran said it would retaliate and days later three Iranian vessels tried to block a British-owned tanker passing through the Strait of Hormuz. But on that occasion, the Iranian vessels backed off when confronted by a British Royal Navy warship.
"We are urgently seeking further information and assessing the situation following reports of an incident in the Gulf," a spokesman for Britain's Ministry of Defense said on Friday.
A person familiar with the matter said a meeting of the British government's emergency committee was being held.
Prime Minister Theresa May's office declined to comment.
Refinitiv data showed the Stena Impero was a British-flagged vessel owned by Stena Bulk.
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.
