“Vessels sometimes shut their AIS when they are going into pirate waters for example, as they wouldn’t want anyone to pick up their signal because they could then be targeted by pirates,” he said.

Straightforward breaks in satellite signals have also been seen in the east Mediterranean.

An Iranian tanker that loaded a cargo of gasoline in Syria transmitted a message on tracking systems that it was heading for Libya in early November.

Satellite tracking showed the ship, the Alvan, sailed west towards Libya before dropping off the radar for at least 24 hours.

When it began to transmit again, it was sailing back in the opposite direction, east towards the Suez Canal, eventually returning to Iran in mid November.

Libyan authorities say they have not engaged in any oil trade with Iran, and no Iranian tankers had passed through Libyan ports.

“I assure you we never received any Iranian vessels in our oil terminals to load or unload,” said Ahmed Shawki, the head of marketing at Libya’s National Oil Corporation (NOC).

(Additional reporting by Jonathan Saul, Amena Bakr and Fumbuka Ng’wanakilala; Editing by Will Waterman)