On Wednesday, armed men broke into an oil services base in Bayelsa state, using dynamite to blow up part of an office building.
The site was operated by Daewoo Engineering and Construction company.
Daewoo later confirmed that nine of its employees, all South Koreans, had been captured.
The Movement for the Emancipation of the Niger Delta, which is fighting for local control of oil assets and reparations for neglect and pollution in the delta and which has conducted kidnapping operations in the past, said it was not involved in the attack.
The delta area accounts for all of Nigeria's oil production but residents say the region has been polluted and is neglected by the authorities.
It is estimated that attacks and kidnappings in the delta have shut down about a fifth of Nigeria's output capacity.