Hostage takers freed a French oil worker captive in Nigeria's volatile southern oil region, releasing the last-known expatriate taken in a spate of seizures this year, officials said Friday.

A military spokesman in the Niger Delta region, Sagir Musa, had no further details on the release of the worker for French oil giant Total SA.

Over 60 foreigners have been seized this year but none are known to remain in captivity after Friday's release. Militants have vowed more seizures, however.

More than one year of stepped-up violence in the Niger Delta has cut daily output in Africa's biggest crude producer by nearly one quarter. Over 100 foreigners have been seized, with scores taken this year alone.

On Thursday two Italians working for the Italian oil firm Agip were freed after a militant group demanding a greater share of the region's oil wealth held them for 99 days.

Kidnapping has become increasingly common in the turbulent Niger Delta region, a vast wetland of swamps with a vast maze of oil pipelines. Despite generating tens of billions of dollars in oil revenues every year, the majority of people remain deeply impoverished and hostages are often taken by militant or criminal groups for publicity or for lucrative ransoms.

Hostages are usually released unharmed after a ransom is paid, although two foreigners died last year in gunbattles between attackers and security forces.

A year of intensified violence in the Niger Delta, where all of the crude in Africa's biggest oil producer is pumped, has helped send oil prices toward new heights on the international markets and sparking severe electricity shortages within Nigeria.

On Thursday, the militant group that held the Italians warned that it would resume its campaign of car bombings — which has cost two lives so far — and renew attacks on foreign workers.

Violence has been rising across the country ahead of next month's elections as politicians pay armed gangs in an effort to win lucrative government posts.