Luanda is the capital of Angola and lies south of the Gulf of Guinea, where Nigerian-based pirates have been operating. If the missing MT Kerala tanker has been taken by pirates, it is the farthest south that Nigerian pirates have struck for refined cargo theft.

The possible theft would mark a particularly bad run for Dynacom Tankers, the company which owns MT Kerala. It also owned the last vessel to be released by Somali pirates in 2013.

Communication was lost with MT Kerala after a suspect large tug boat was sighted in a restricted area offshore Angola on January 17, reportedly close to the anchored position of the tanker.

Already this year, the MT Super League tanker was boarded off the coast of Equatorial Guinea's boarder with Gabon. This was then followed by the hijacking and kidnapping of three crew members from cargo vessel MV San Miguel, also off Equatorial Guinea.

"We have been watching Nigerian based pirates launch an increasing number of attacks on vessels in areas not normally associated with piracy of late," Dryad Maritime director of intelligence Ian Millen said.

"If substantiated, this latest incident demonstrates a significant extension of the reach of criminal groups and represents a threat to shipping in areas that were thought to be safe."

According to the maritime intelligence agency, if the MT Kerala was taken for refined product cargo theft, the pirates would most likely have forced the ship master to navigate to a location, normally offshore Niger Delta, where a portion of its cargo would be siphoned off to a smaller vessel, before MT Kerala and its crew were released.

"The criminal gangs that conduct this particular brand of intelligence-led maritime crime are well-prepared, well-armed and have specialist maritime knowledge and expertise," Millen added.

"Operations are primarily targeted at ships in offshore anchorages, sometimes during ship-to-ship cargo transfer ops with attacks mainly conducted under cover of darkness."

"The criminals usually disable communications and switch off the automatic identification system to avoid being detected, meaning that the first indication that owners have of the hijack is normally when they lose contact with the ship."