The men, seized from the offices of China's National Petroleum Company, were freed after "complex efforts", China's foreign ministry said.

All nine were safe and well and were handed back to their company in Bayelsa state, a ministry statement said.

About 100 foreign hostages, mostly oil workers, have recently been kidnapped in the oil-producing Niger Delta.

Many Niger Delta residents live in poverty, and taking hostages for ransom has become increasingly common.

Hostages are usually released after a ransom is paid, though some have been killed in gun battles between security forces and kidnappers.

There was no word about whether a ransom was paid to secure the release of the nine men.

Their release comes after the freeing of five Chinese telecoms who were held for 12 days in captivity.

China's President Hu Jintao is currently on an eight-nation tour of Africa aiming to boost trade ties with the continent.

Foreign workers from the United Kingdom, the United States, the Philippines, Italy, Lebanon and Nigeria are currently being held hostage in Nigeria.

The instability in the region has cut Nigeria's oil production by at least 20%, costing the country some $4.4bn (£2.2bn) last year, according to the government.