BUJA - Pirates have released five Polish and Russian crew members they kidnapped from a cargo ship off the coast of Nigeria last month, the vessel's management company said.Piracy is increasing in the west Africa's Gulf of Guinea, an important exporting region for oil, cocoa and metals, and insecurity is driving up shipping costs.The hostages were taken when the Antigua and Barbuda-flagged MV City of Xiamen container ship was attacked late on April 25 about 100 miles (160 km) off Nigeria's coast.