Nigeria's army is using helicopters to search the creeks around the southern city of Port Harcourt for the group which attacked an oil firm on Tuesday.
There have been several attacks on oil companies in the area
Despite the massive manhunt, no arrests have been made yet.
A BBC correspondent in the region says many employees of Italian oil firm Agip have not turned up for work after armed men in speedboats attacked its office.
It is the latest in a string of attacks on oil companies in the Niger Delta, home to most of Nigeria's oil wealth.
Four foreign oil workers were kidnapped in the area, by an armed group demanding the release of two ethnic leaders and more local control of oil resources.
It is not clear which group carried out Tuesday's attacks on the Agip offices.
At least nine people were killed in the Port Harcourt assault on Agip's riverside offices, reportedly including eight policemen.
Our correspondent, Abdullahi Kaura Abubakar, says many Agip workers are in shock.
The gunmen reportedly managed to steal tens of thousands of dollars from a bank located in the riverside offices but our correspondent says they missed out on an even larger haul.
A large deposit was due to be made on Tuesday but the raid took place before the van carrying the money had arrived.
The company said it had "temporarily evacuated staff and contractors from the area of the base affected by the incident and the situation is currently under control".
According to a government official who spoke to the Reuters news agency on condition of anonymity, a group of about 30 men, wielding AK-47 assault rifles, attacked the facility by speedboat.
Local resident Damka Pueba arrived at the scene shortly after the shooting stopped.
She described scenes of chaos to the BBC News website. Staff were fleeing the building as local police ran inside and hundreds of anxious relatives gathered in hope of news about their loved ones.
"A crowd of about 800 people had gathered at the scene. Many of them were crying," she said.
It is unclear if robbery was the sole motive for the latest attack.
It comes just days after militants, who have kidnapped four foreign oil workers and attacked a Shell oil platform, said they were preparing to carry out more raids.
The foreign hostages have been captive for more than two weeks.
The Nigerian authorities say they have made some arrests in connection with the case but the kidnappers are still able to send e-mails to news agencies.
The American hostage had been reported to be unwell but in an e-mail to the AP news agency, the kidnappers say all four are in "good health".
They "have adapted fairly well to the conditions under which the people of the Niger Delta have been kept for the last 48 years," the group said.
Oil workers' unions in Nigeria have threatened to withdraw members from the main oil-producing region unless the government moves to improve security.
The instability has led to a 10% fall in Nigeria's oil production. The country is Africa's leading oil exporter and the fifth-biggest source of US oil imports, but despite its wealth, many Nigerians live in abject poverty.