Police in Nigeria's main oil-producing region have discovered 12 bodies left in a village and rescued a Romanian expatriate worker kidnapped in separate incidents, authorities said on Friday.

Police on patrol on Wednesday "saw the corpses of 12 persons, suspected murdered, with their hands tied at the back," said Suleiman Abba, police commissioner for Rivers' State, located in the Niger Delta region.

The yet-to-be identified bodies were discovered in Ula Ikata village, around 100 kilometres (60 miles) from the state capital of Port Harcourt. No arrests have been made.

Meanwhile, police said a joint security team including the military and intelligence agents on Thursday rescued a Romanian engineer along with six Nigerian workers who had been kidnapped earlier in the week in the same state, Abba said.

The Romanian had been working for an electricity consultancy firm. It was not clear if a ransom had been paid, but two suspects were arrested.

Criminal gangs in the region carry out abductions for ransom, with expatriates and wealthy locals as main targets.