Nigerian President Muhammadu Buhari has ordered an investigation into communal clashes, which have left hundreds dead in central Benue state.
Mr Buhari expressed "deep shock" at the past week's violence between herdsmen from the Fulani ethnic group and local farmers, an official statement said.
Several thousand people have been displaced, according to local media.
Benue has a history of violent attacks and reprisals between semi-nomadic herdsmen and farmers.
The clashes are often linked to cattle raiding.
Mr Buhari called for unity among Nigerians, saying: "There should not be any reason why Nigerians of any group or tongue cannot now reside with one another.
"Different groups of Fulani militants killed a total of more than 1,200 people in 2014, meaning that if taken together they would be the world's fourth deadliest militant group, according to the most recent Global Terrorism Index.
The scope of their attacks is now enough to "pose a serious threat to stability", the report said.
The communal violence in central Nigeria is not connected to the six-year insurgency by Islamist militant group Boko Haram in the country's north-east.
Boko Haram was the world's most deadly militant group, according to the report.