The region has seen 31 incidents, including four hijackings, in the opening six months of this year as gangs turn their attention to ships other than offshore vessels and product tankers.

According to the International Maritime Bureau armed pirates off the Gulf of Guinea took 56 sailors hostage and were responsible for all 30 crew kidnappings reported globally so far in 2013.

Capt Pottengal Mukundan, director of the IMB, said in a statement: “There has been a worrying trend in the kidnapping of crew from vessels well outside the territorial limits of coastal states in the Gulf of Guinea

“In April 2013, nine crew members were kidnapped from two container vessels, one of which was 170 nautical miles from the coast.

“Pirates have used motherships, some of which were smaller off-shore supply vessels hijacked by pirates to conduct the attacks. There continues to be significant under-reporting of attacks – a phenomenon highlighted by the IMB year on year.

“This prevents meaningful response by the authorities and endangers other vessels sailing into the area unaware of the precise nature of the threat.”

Attacks off Nigeria accounted for 22 of the region’s 31 incidents and 28 of the crew kidnappings, the IMB says.

There is some good news with attacks of Somalia reaching the lowest level since 2006 during the first half of this year.

Just eight incidents were recorded in the region during the period, although two led to the vessels being hijacked, the report says.

Globally piracy incidents were down from 138 in the first half from 177 in the same stretch of 2012.