Militants in southern Nigeria's oil-rich delta have kidnapped 10 oil workers on Friday, but released three of them later in the day.
The militants invaded the offshore platform of oil company Conoil in several speedboats on Friday and fired gunshots sporadically for some time before abducting their victims.
Late on Friday they released a Filipino, a South African and a Nigerian. Still being held were three US nationals and four Britons, Bayelsa State government spokesman Joshua Benamisia said.
Those taken hostage were working for Transcoastal Oil Servicing Company, a contract firm engaged by Conoil, an indigenous Nigerian company.
"They overpowered security operatives at the platform who had inferior fire power before taking the hostages," Benamisia said.
Police had begun to search the militants and their victims, a spokesman for the Bayelsa State police said, though no arrest had been made and no group had claimed responsibility for the abduction.
Hostage takings have become increasingly common in the West African nation's Niger Delta, where militants seeking a greater say over their region's oil wealth have staged kidnappings and attacks on oil facilities to draw attention to their cause.
More than 100 foreigners and Nigerians have been seized this year. Hostages are usually released unharmed after a ransom has been paid.
Nigeria is Africa's largest oil producer and derives some 90 per cent of its annual income from the resource.