Pirates have hijacked a ship with 23 crew members in the Gulf of Guinea, which has become an increasingly dangerous area for international shipping.

The International Maritime Bureau says the ship was seized Wednesday about 115 kilometers southwest of Benin's capital, Cotonou.

The bureau's Piracy Reporting Center says armed pirates boarded a product tanker, took the crew members hostage, and sailed the vessel to an unknown location.

It says pirates boarded another product tanker in the same area, but departed after the crew members locked themselves in the engine room.

The center later reported a failed robbery attempt on another ship about 12 kilometers off the coast of Togo.

Experts have warned that pirate attacks are increasing off the coast of West Africa, mainly near Benin. They note that pirates in the area tend to attack ships for their cargo, rather than hold the crews for ransom.

Pirates on the other side of Africa, in Somalia, usually demand ransom payments for the release of ships and crews.

The International Maritime Bureau says there have been more than 320 pirate attacks worldwide this year and at least 33 hijackings. The majority of the attacks have happened in waters near Somalia.