Twelve workers from Ukraine and Russia were on board a boat that was attacked while in international waters off the Niger Delta, but seven have been freed.
The boat, which belongs to the Saipem oil services company, was attacked about 85 nautical miles from the coast, after it had left its naval escort.
A spate of attacks on the oil sector has cut Nigeria's production by 25%.
The boat has also been freed and is on its way back to the region's main city, Port Harcourt, the officials say.
This is one of the attacks carried out furthest from Nigeria's coast, correspondents say.
Until recently, they have usually operated on land or in the creeks of the Niger Delta.
Nigeria's oil militants say they are campaigning for more of the country's oil wealth to be used to benefit residents of the Niger Delta.
But correspondents say there are also many criminal gangs motivated by the ransom money often paid by oil companies to secure the release of their workers.
Earlier this week, the head of Nigeria's state-owned oil company told a parliamentary enquiry it had paid $12m to oil militants to prevent them attacking a pipeline.
Source : BBC News