ABUJA (Reuters) - Pirates have kidnapped five crew members from a cargo ship off Nigeria, a piracy watchdog and a security source said on Monday, part of a growing trend of attacks that are pushing up maritime insurance costs in the Gulf of Guinea.

The Antigua and Barbuda-flagged MV City of Xiamen container ship was attacked late on April 25 off the coast of oil-producing Bayelsa state, a security source said.

The crew were mostly Sri Lankans but included one member from Russia and another from Myanmar, the source said.

The International Maritime Bureau (IMB) said 14 heavily armed pirates attacked the container ship, breached its citadel - a strong room designed to protect the crew from attack. The pirates took five crew captive before escaping with cash taken from the ship and the crew, the IMB said in a piracy report.

Pirates in a speed boat off Nigeria chased and fired upon another container ship on April 24 but the vessel increased speed and escaped, the IMB said.

West Africa is an important source of oil, cocoa and metals for world markets, but international navies are not engaged in counter-piracy missions at present. Many vessels are forced to anchor off Nigeria with little protection, making them a soft target for criminals.

Naval patrols and the presence of armed guards aboard merchant vessels have helped reduce piracy off Somalia on Africa's eastern coast.

(Reporting by Joe Brock; Editing by Tim Cocks and Jon Boyle)