The Movement for the Emancipation of the Niger Delta demand profits from oil stay in Nigerian hands and told oil companies, including Shell for whom the kidnapped employees work, to leave the region by Saturday.
The BBC reports the workers were on a Shell oil barge on the Niger Delta when they were taken hostage.
The rebels also set a Shell facility on fire.
Nigeria is the leading oil exporter in Africa and the fifth-largest source of U.S. imported oil but most Nigerians live in poverty.
The group has attacked two oil pipelines and two oil fields and took four other foreign workers hostage in recent weeks.