Piracy in the Gulf of Guinea region, which includes Africa's biggest oil producer Nigeria, is pushing up costs for shipping firms operating there. Many experts believe the region's pirate gangs grew out of insurgent groups involved in oil theft in Nigeria's restless Delta region.

Pirates raided the Liberia-flagged ship, the Hansa Marburg, on April 22.

"The four seafarers, who were taken from the vessel by armed men 130 miles southwest of Malabo, Equatorial Guinea, and held hostage, have now been released," Hamburg-based shipping firm Leonhardt and Blumberg said in a statement.

The company said the crew - two Ukrainians, one Russian and one from the Pacific island nation of Kiribati - were in good spirits, but gave no further details of their release to "avoid encouraging further criminal acts of this kind".

The region is an important source of oil, cocoa and metals for world markets. International navies have not launched counter-piracy missions in the Gulf of Guinea, unlike in Somalia, where piracy was once rampant and has been largely brought under control.

Many vessels are forced to anchor off regional ports with little protection, making them soft targets for criminals.

(Reporting by Joe Bavier; additional reporting by Jonathan Saul in London; editing by Tom Pfeiffer)