Pirates in Nigeria's restive oil-producing Niger Delta region have heightened their attacks on oil workers and authorities in the region, killing three policemen and kidnapping nine people since last Thursday, according to security officials.

In one attack, gunmen travelling by boat shot and killed three policemen who were escorting an Italian vessel operated by the oil company ENI on the Santa Barbara River.

"Sea pirates attacked and killed three of our men. They were escorting an Agip (ENI) barge when they were attacked. We have recovered their bodies and the gunboat," Valentine Ntomchukwu, police commissioner for the Bayelsa state, told Reuters.

A security source told Reuters that the driver of the boat had been kidnapped by the assailants. Representatives from ENI were not available for comment.

In other attacks, six workers from a local oil services company were kidnapped by unidentified gunmen in the Nembe local government area, and the master and chief engineer of a supply vessel were kidnapped by seven pirates who approached by speedboat.

Piracy in the Niger Delta is increasing despite a worldwide fall in attacks. Global piracy reached its lowest levels in six years in 2013 with only 264 attacks recorded, according to the International Maritime Bureau (IMB). As of 16 September, only 163 incidents were reported to the organisation during this year.

The region has witnessed decades of conflict as the Niger Delta's ethnic minorities feel that they do not receive their fair share of oil money garnered by the Nigerian government.

The Movement for the Emancipation of the Niger Delta (Mend) seeks a fairer distribution of this money for the impoverished who live in the region and have carried out a number of kidnappings, sabotage, as well as attacks on representatives of oil companies and Nigerian authorities.