By Tife Owolabi
YENAGOA, Nigeria (Reuters) - Four South Koreans and a Nigerian who were abducted earlier this week in the oil-producing Niger Delta have been released, police said on Saturday.
The four foreign hostages were handed over to South Korean officials in the West African country late on Friday after police arrested suspects in the case.
"The victims were picked up from Azikoro village by men of the special security outfit at about 9 p.m. (2000 GMT)," Bayelsa Police Commissioner Kings Omire told Reuters.
Police spokesman Fidelis Odunna said: "They were released voluntarily because we had suspects in our custody and owing to a hit on their camp, they had to let the men go."
Omire said another Nigerian taken had already been released, meaning all six who were abducted were now free. The two arrested suspects were being interrogated, he said.
There was no immediate comment from either the South Korean government or Hyundai Heavy Industries, the conglomerate that employs the abducted workers.
Kidnapping is rife in Africa's top oil producer, making millions of dollars a year for criminal gangs. It is common across the south, especially in the Niger Delta.
Gunmen abducted two Lebanese men working for Nigerian construction company Setraco in Delta state this month, and killed a soldier protecting them.
The 83-year-old mother of Finance Minister Ngozi Okonjo-Iweala was kidnapped on December 9 in Delta state but was freed five days later after a military search.
In the north of Nigeria, where Islamist militants operate, another form of kidnapping of foreigners has emerged this year.
A French national working for renewable energy company Vergnet was abducted close to the border with Niger on Wednesday.
French intelligence agency DCRI believes the kidnappers were linked to "terrorist activity". Islamist groups linked to Boko Haram have been behind similar kidnappings.
Boko Haram, which wants to impose strict sharia (Islamic) law in a country with a mixed Christian and Muslim population, has killed hundreds in an insurgency this year in northern Nigeria.
(Writing by Joe Brock; Editing by Mark Trevelyan)