LAGOS (AFP) -- Unidentified gunmen shot dead four policemen in an ambush on a bank bullion van and made away with several million naira, the Nigerian currency, near the oil city of Port Harcourt, police said Tuesday.

"We lost four officers in the attack at Emuohua in Port Harcourt yesterday," a senior police officer told AFP by telephone.

He said the bandits stole several million naira the bank was taking to Yenagoa, the capital of Bayelsa State. He said the police had launched a manhunt for the fleeing robbers.

The past months have seen a wave of bank robberies in and around Port Harcourt, the centre of Nigeria's multi-billion-dollar oil and gas industry.

The police are looking for a Niger delta militant, Akete Tom, for alleged involvement of his gang in bank robberies in the city.