Four foreign workers, including two Britons have been taken by a group of armed men from a nightclub in Port Harcourt in southern Nigeria on Sunday.
It is the fifth kidnapping in the oil-rich Niger Delta in two weeks. State police have made no arrests.
In most cases, hostages are released unharmed, usually after a financial deal is struck, but analysts say the ransom payments fuel the violence.
On Monday, three Filipinos were released 10 days after being kidnapped.
A rise in attacks in recent months by militants, seeking more local control of the Niger Delta's rich oil resources, has cut Nigeria's oil production by 25%.
A British Foreign and Commonwealth Office spokesperson said they were in contact with the Nigerian authorities, and the companies the men work for will be in touch with their families.
Reports say gunmen entered the Goodfellas bar in Port Harcourt popular with foreign oil workers and went around asking people their nationality and who was their employer. A barman said they took away white people.
Militant attacks have cut Nigeria's oil production by 25%
"They were shooting and everyone started screaming," driver George Ani told AP news agency.
"They took some expatriates but I don't know how many. I lay on the floor of my car until it was finished."
Blood stains were visible on the floor.
"The kidnappers ... burnt one of the vehicles they used, maybe to destroy anything that could give them out, and escaped through the waterway," a police spokeswoman in Port Harcourt told Reuters news agency.
Industry sources say hostage-taking has become an attractive business.
The Delta is awash with weapons, unemployment is high and communities feel aggrieved at the lack of development.
Armed groups have proliferated, often linked to local politicians.
The BBC's Alex Last in Nigeria says the money is often used to buy weapons and there are fears that the various often competing groups are trying to strengthen their positions ahead of elections early next year.
The three Filipinos were freed on Monday and handed over to the Philippine embassy.
They had been working at the Nigeria Liquefied Natural Gas complex in Bonny.
There is no news on a German oil worker who was taken at the same time.
Last Wednesday, two Norwegians and two Ukrainians were seized from a ship offshore.
On Thursday, a Belgian and a Moroccan were abducted while travelling in a car.