A piracy monitoring group says pirates have hijacked a ship with 23 crew members off the coast of West Africa.

The International Maritime Bureau says the attack happened Wednesday about 60 kilometers southwest of Benin's capital Contonou.

The report says armed pirates boarded and seized control of 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 location, but left the vessel after the crew of that ship locked themselves in the engine room.

Piracy experts have warned that attacks are increasing off the coast of Benin and nearby countries. The IMB warns that West African pirates tend to be violent and advises ships to be extra cautious when traveling in the Gulf of Guinea.

The organization says there have been more than 320 pirate attacks worldwide this year and at least 33 hijackings.

The majority of the attacks happen in waters near Somalia, where pirates have taken hundreds of millions of dollars in ransom payments over the past few years.