The International Maritime Bureau says pirates have seized a chemical tanker and kidnapped its crew off the coast of Nigeria.

IMB official Cyrus Mody said Tuesday the pirates attacked the Marshall islands-flagged vessel on Saturday about 145 kilometers south of Lagos, Nigeria's commercial capital.

Mody said the ship is still under pirate control, but he believes all crew members are okay. It is not known how many people are on board the ship.

Pirates have been carrying out attacks in West Africa's Gulf of Guinea with greater frequency, often targeting vessels carrying oil from Nigeria's oil-rich southern Delta.

Pirates recently hijacked several vessels off the coast of Benin, including at least two oil tankers.

In June, the International Maritime Bureau issued a piracy warning for ships traveling near Benin.