Warri, Nigeria: Gunmen in Nigeria's restive oil-rich south have killed two police guards and kidnapped two Shell workers, police said Saturday.

The violence is the latest in a long line of attacks in Nigeria's southern Rivers state, where kidnapping for ransom is rife.

"Efforts are in top gear towards ensuring that we get them out from the custody of their abductors," Rivers State Police spokesman Omoni Nnamdi said.

The attack took place on Thursday afternoon in the Rumuji district of Rivers state, a short distance northwest of the key oil city of Port Harcourt, as the team returned from visiting areas of Bayelsa state.

In past such attacks, the victims have often been released after a few days once ransoms are paid.

The Royal Dutch Shell oil workers were escorted by police in a pickup truck, but gunmen reportedly ambushed the convoy by opening fire at the guards to stop the vehicles.

Two policemen were killed, while gunmen dragged out the workers from their vehicle.

The Shell Petroleum Development Company of Nigeria Limited (SPDC) confirmed the attack on its staff and police escort, but gave no further details. It was not clear what the nationalities are of those who have been kidnapped.

Elsewhere in Rivers state, gunmen on Thursday also ambushed a 30-seater bus, killing the driver and abducting more than a dozen passengers - but police said all the hostages were freed safely soon after.