All the vessels in question were registered in Tanzania.

“In the past months we witness a recurring pattern of vessels sailing the Tanzanian flag that transmit the same MMSI number (a satellite signal that provides information on a ship’s identity and position),” said Windward, a firm that provides maritime analytics technology.

“This way, if one of the two vessels is engaged in legitimate maritime activities, it might be used as a ‘cloaking’ for the other vessel and its activities.”

Iranian oil tanker Millionaire sent messages that doubled over a voyage made by a Syrian-owned ship, the Lady Rasha.

In a separate instance, the satellite tracks of Iranian oil tanker Pioneer were mixed up with a Tanzania-flagged cargo ship called the Talavera, recently renamed Chief Ahmed, and travelling from the Mediterranean into the Red Sea.

Despite all the paired vessels appearing to be registered under Tanzanian flags, officials in mainland Tanzania and Zanzibar denied holding any information on the vessels.

They have directed queries to a shipping agency in Dubai, Philtex Corporation, which they say registered some Iranian ships under the Tanzanian flag without their knowledge.

Philtex confirmed it had registered the Syrian-owned Lady Rasha, but could not provide details on the Iranian tankers in question.

MYSTERY CRATES