ABUJA (Reuters) - Nigeria's multiple security problems have created a crisis that requires urgent attention and could lead to instability in other African countries if it is not addressed, a United Nations rapporteur said on Monday.

Security forces in Africa's most populous country are trying to tackle a decade-long Islamist insurgency in the northeast, banditry in the northwest and bloody clashes between nomadic herdsmen and farming communities over dwindling arable land in central states.

Agnes Callamard, the U.N. special rapporteur on extrajudicial, summary or arbitrary executions, said Nigeria was a "pressure cooker of internal conflict".

"The overall situation I have found is one of extreme concern," she told a news conference in the capital, Abuja, where she presented her preliminary findings following a 12-day visit to the country.

Callamard said the police and military had shown an excessive use of lethal force across the West African country which, combined with a lack of effective investigations and meaningful prosecution, caused a lack of accountability.

She said the country required changes in the judiciary, police and military to stop people resorting to violence in the absence of justice.

"The lack of accountability is on such a scale that pretending this is nothing short of a crisis will be a major mistake. If ignored, its ripple effect will spread in the sub-region given the country's important role in the continent," she said.

Spokesman for the ministries of justice, military and police did not respond to a Reuters request for comment on Callamard's findings.

The Islamist insurgency waged by Boko Haram began in northeast Nigeria in 2009 but has spread to parts of neighboring Cameroon, Chad and Niger where members of the group and militants allied to Islamic State carry out attacks.

The rapporteur also condemned what she said was the "arbitrary deprivation of life" and the excessive use of lethal force in the case of processions held by banned Shi'ite Muslim group the Islamic Movement in Nigeria.

Callamard said the move to ban the group appeared be based on what the authorities thought IMN could become rather than its actions. She said she had not been presented with any evidence to suggest the group was weaponized and posed a threat to the country.

(Reporting by Alexis Akwagyiram and Camillus Eboh; Editing by Peter Cooney)

I saw this on Aljazeera News on Monday......the report gave the
Nigerian Government & police a heavy knock on the head........so many human rights issues recorded, unwarranted arrest of protesters, use of excessive force on protesters by the police, selective fight against "supposed curruption"......I just dey shame for Buhari & his crew as Aljazeera dey report about Nigeria on tv.........but hey, his zombies will not see anything wrong with APC leadership.......they would rather blame it on PDP..........Next Level of blame game.

Next level (4+4). APC....killed them all.
Irresponsible National Electoral Commission brought this regime to power by helping Buahri to rig the election. 70% of Nigerians did not vote for the outdated president in ASO Rock.