Unknown gunmen have killed 13 people in an attack in central Nigeria, police said on Monday, in an area known for ethnic clashes.A local resident said the assailants opened fire late Sunday on a bar where people were drinking beer in the village of Kwatas, 80km south of the city of Jos. "Thirteen persons lost their lives while five persons were critically injured and are receiving treatment," regional police spokesperson Ubah Agaba said. The reasons for the attack were not immediately clear.Central Plateau state is part of Nigeria's so-called middle-belt that divides the mainly-Muslim north from the predominantly Christian south.Deadly clashes between nomadic Fulani herders and farmers over land, grazing and water have plagued the area for years.Local resident Usman Hardo said that some hamlets where Fulani live were burnt down following the attack. State governor Simon Lalong said his "heart again bleeds" for the victims of the latest killings as he called on the security forces to find those responsible. "We do not need another circle of bloodshed," he said in a statement.Aid group Mercy Corps said in May last year that violence between farmers and pastoralists in Nigeria had "contributed to more than 7 000 deaths in the past five years".Attacks in remote areas often go unreported but the authorities in Plateau state insist the bloodshed has decreased following reconciliation efforts between the communities.Gunmen on January 8 killed 12 people in a village some 25km away from the scene of the latest incident.