Abuja - Nigerian authorities have largely ignored sectarian clashes in the nation's religiously mixed central region that have killed 3,000 people since 2010, Human Rights Watch said on Thursday.
Local police rejected the findings by the international watchdog, which said that a series of massacres and tit-for-tat sectarian attacks have gone largely unpunished as police overlooked witnesses or failed to collect evidence properly.
"Witnesses came forward to tell their stories, compiled lists of the dead, and identified the attackers, but in most cases nothing was done," said Daniel Bekele, Africa director at Human Rights Watch.
"The authorities may have forgotten these killings, but communities haven't. In the absence of justice, residents have resorted to violence to avenge their losses," he said of a new 146-page report entitled 'Leave Everything to God'.
Africa's second-biggest economy and top oil exporter is growing as an investment destination but reports of violence and corruption by authorities are tarnishing its image.
The report was based on interviews with 180 witnesses and victims in Kaduna and Plateau states, which lie in Nigeria's volatile "Middle Belt", where the largely Christian south meets the mostly Muslim north.
Plateau Police Commissioner Chris Olagbe rejected the findings.
"That is totally untrue and unholistic," he told Reuters.
"All the gunmen that have been arrested in Plateau have been taken to court," Olagbe added, without giving details of any convictions.
Plateau state government spokesman Yiljap Abraham said Human Rights Watch had reported hearsay and its reports were "not objective or balanced".
The Kaduna police commissioner said he would not comment until he had seen a copy of the report.
Last year, incoming Police Inspector General Mohammed Abubakar said Nigeria's force had fallen to its lowest level with officers perverting justice, locking up innocent people and carrying out torture and extra-judicial killings.
President Goodluck Jonathan ordered an overhaul of the police in February 2012 when appointing Abubakar and the police chief has claimed improvements have since been made.
Nigeria is Africa's most populous nation with almost 170 million people, split roughly equally between Christians and Muslims, although there are around 250 different ethnic groups who mostly live peacefully side-by-side.
The central region has been a tinderbox for sectarian violence for years, mostly stirred by resentment between settler Christian communities and nomadic Muslim Fulani.
Jos, the capital of Plateau state, was at the centre of some of the worst recent violence when around 1,000 people were killed in a series of massacres in 2010.
Post-2011 election violence in Kaduna state resulted in around 800 deaths in three days, in what began as northern grievances about perceived political alienation but quickly morphed into ethnically-focused attacks.
The unrest risks drawing in Islamist sect Boko Haram, an al Qaeda-linked group which wants to impose sharia (Islamic) law in northern Nigeria. Most of the sect's attacks are contained further north but it did claim a 2011 Christmas Day bomb attack at a church in Jos.
Human Rights Watch found that although police often make mass arrests following clashes, no legal process was followed which would enable a prosecutor to link suspects to crimes.
"In the vast majority of cases, the authorities will quietly drop the charges," the report said. Some police will not investigate a case unless the complainants pay them, while others don't want to arrest suspects for fear of sparking further violence.