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 pirates seized the Cypriot-flagged tanker Wednesday about 115 kilometers southwest of Cotonou, the capital of the West African nation of Benin.

The bureau's Piracy Reporting Center says armed pirates boarded the ship, took the crew hostage, and sailed off towards an unknown location. The nationalities of the crew are unknown.

The center says pirates boarded a Norwegian-flagged ship in the same area, but failed to seize control after the crew members locked themselves in the engine room.

Experts have warned that pirate attacks are increasing off the coast of West Africa, mainly near Benin. They say there have been 19 acts of piracy off Benin so far this year, compared with none in 2010.

The experts note that pirates off West Africa tend to attack ships for their cargo while pirates from East Africa, usually Somalia, hold the crews for ransom

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