The gunmen, and one member of the security forces, were killed in a gun battle at sea, Major General Sali Mohamadou, commander of the second military unit in Douala, told state radio. A security source said the clash took place on Saturday as the gang neared the Nigerian border.

Up to nine people were killed in the bank raid on Friday night, the latest in a series on attacks by seaborne gunmen in the Gulf of Guinea, which produces three million barrels of oil per day but has pockets of insecurity.

“This is a warning to all that anyone who dares such an act in Cameroon again will not run away. We will always catch up with them,” Mohamadou said.

Mohamadou did not say how much money was stolen from the branches of Ecobank and First Trust Bank. Two gunmen were being questioned, he added.

A security source told Reuters that a gang of about 25 arrived in a Douala suburb by speedboat before blowing their way into the banks and making off with an undisclosed amount of cash.

State radio reported that five people were killed in the bank raid but the security source put the toll at nine.

The incident comes days ahead of a US -led multinational maritime exercise planned off Cameroon’s coast, part of broader efforts to improve security.

While piracy on the east coast of Africa is not on the scale as off Somalia, there are fears it is on the rise in a region poorly equipped to handle the threat.