Iran’s Revolutionary Guards said on Friday they had captured a British-flagged oil tanker in the Gulf after Britain seized an Iranian vessel earlier this month, further raising tensions along a vital international oil shipping route.

Britain said it was urgently seeking information about the Stena Impero after the tanker, which had been heading to a port in Saudi Arabia, suddenly changed course after passing through the Strait of Hormuz at the mouth of the Gulf.

The Revolutionary Guards said they seized the tanker at the request of Iranian maritime authorities for “not following international maritime regulations,” state television reported.

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.