A maritime monitoring group said Wednesday that pirates have kidnapped at least two members of a cargo ship anchored off the Nigerian coast.

The International Maritime Bureau said eight Nigerian pirates opened fire on the Dutch-owned vessel Tuesday, robbing the 14-member crew before escaping in a small speedboat with the ship's captain and engineer.

It said an additional crew member was missing following the attack and another suffered unspecified injuries.

Pirates in the Gulf of Guinea have become increasingly bold in recent months. IMB says there has been seven reported pirate attacks in the region this year.

Two weeks ago, pirates carried out a deadly hijacking on a cargo ship off the coast of Nigeria, killing the ship's captain and chief engineer.

IMB has warned vessels in the region to keep strict anti-piracy watch, saying at least one group of pirates appears to be preying on ships in the area.

West African pirates are not as active as their counterparts in Somalia, but a rising number of attacks prompted the IMB to post warnings about traveling through the Gulf of Guinea.