In a separate incident in southern Nigeria's Rivers state last month, five Chinese telecommunication workers were kidnapped and safely returned within two weeks.


As Chinese travel abroad in ever-greater numbers, they are increasingly falling victim to crime, kidnappings and murder, particularly in dangerous and unstable parts of Africa and Asia where Chinese companies have sought contracts.


Foreign workers, especially in the oil industry, have become targets of armed fighters seeking a greater share of Nigeria's oil wealth.


"We hope overseas Chinese enterprises and personnel will further raise their awareness of safety and risk and step up precautionary measures so as to ensure their safety," Jiang said.


The release of the nine workers comes as Hu Jintao, the Chinese president, embarks on an eight-country tour of Africa aimed at boosting trade ties, though Nigeria is not on the itinerary.