At least 16 people have been killed by gunmen at a church in southern Nigeria after a New Year's Day service, police have said, with some local reports putting the death toll as high as 21.

Police told the BBC that the group had attended a midnight service before they were ambushed in the early hours of Monday.

The incident, which happened in the oil-rich region of Rivers state, has been linked in local media reports to growing tensions between rival gangs.

The gunmen are said to have fired at random and killed some at close range.

Reports said that the noise of fireworks for the New Year celebrations in Omuku town made it difficult for residents to realise that shots were being fired and people were being killed.

Omoku, which is about 85km (50 miles) from the state capital Port Harcourt, has a history of violent crime and the BBC said that there has been an increase in violence in the past year, after a government amnesty programme for former gang members was allowed to lapse.

The amnesty had reportedly brought relative peace to a region which had been plagued by attacks by militants demanding that a greater share of Nigeria's oil wealth be kept by the country's oil-producing regions.

The Boko Haram extremist group has been terrorising north-eastern Nigeria and border regions for years with at least 20,000 people reportedly dying in their attacks and millions displaced.

However, the group has yet to claim any attacks in the largely Christian south of the country.