A British warship radioed an Iranian patrol boat to warn against interfering with the Stena Impero as the oil tanker passed through the Strait of Hormuz, according to a recording obtained by Reuters.
Iran's Revolutionary Guards seized the British-flagged tanker with a crew of 23 on Friday.
"Please confirm that you are not intending to violate international law by unlawfully attempting to board," Royal Navy frigate Montrose warned the Iranian boat, the recording provided by maritime security firm Dryad Global showed.
The warship also reminded the tanker's crew of their right to pass through the strait: "As you are conducting transit passage in a recognised international strait, under international law your passage must not be impaired, impeded, obstructed or hampered."
In the radio messages the Iranian patrol boat is heard instructing the Stena Impero to alter course.
Responding to the Montrose's warning not to interfere, the Iranian boat says it is taking action for security purposes.
"This is Sepah Navy patrol boat. No challenge is intended, no challenge is intended. I want to inspect the ship for security reason."
The Sepah Navy is a special unit of the Navy of the Islamic Revolutionary Guard Corps.
Almost a fifth of the world's annual oil consumption passes through the Strait of Hormuz, which lies at the entrance to the Gulf between Iran and Oman.
