Local officials in Port Harcourt confirmed the release of the five hostages late Thursday. The Koreans were abducted from a a natural gas plant in the Niger Delta on Wednesday.

Militants wearing black masks, military fatigues and carrying Kalashnikov assault rifles and rocket-propelled grenade launchers patrol the creeks of the Niger Delta area of Nigeria (file photo)

The freed men were said to be in good condition when they were brought to the governor's office in Port Harcourt, shortly after they regained their freedom.

The adduction and release of the five foreign workers reflects the growing insecurity in Nigeria's troubled Niger Delta region, which accounts for nearly all of Nigeria's oil and gas production.

The Koreans were released after a plea from the jailed militant leader in whose name they were abducted.

Abuja-based lawyer and commentator Maxi Okwu warns that the current wave of violence sweeping the Delta is emerging as a national security threat.

"I think Nigeria's democratic progress, or the country's future, now hangs literally on the Niger Delta, the question of the Niger Delta," he said. "We must address it radically, failing which the incessant, or continuing terrorist tactic employed by the restive youths there will continue. They are expressing an anger on the failure of the Nigerian state to provide for their comfort, and take care of the environment."

The Nigerian security forces have not been able to halt attacks on foreign oil and gas workers, which have become more frequent and daring in the past few months.

Analysts say, with more than 3,000 oil and gas facilities spread across 74,000 square kilometers of swamp and forest, the poorly motivated Nigerian military may find it impossible to provide effective security for the industry.

Nigeria's oil output is currently down by about 25 percent, due to attacks by militant groups, particularly the Movement for the Emancipation of the Niger Delta.