Release International, which campaigns for persecuted Christians, has urged newly elected Nigerian president Muhammadu Buhari to protect the country's Christian population against attack.
Christians are under pressure from the Islamist Boko Haram movement and from Muslim Fulani herdsmen. Many Christians believe they are armed and funded by Islamist movements.
The Global Terrorism Index ranks Nigeria as the third most terrorised country for the fourth consecutive year. Between them, Boko Haram and Fulani militants have killed tens of thousands and driven millions from their homes.
'They have killed with impunity, and that impunity must stop,' said Paul Robinson, Release chief executive.
Release says the president's priority must be to protect his nation's vulnerable Christian communities.
Christian leaders say the attacks by heavily armed herdsmen amount to ethnic and religious cleansing. They also accuse the military of standing by and failing to prevent the raids. Some even claim collusion by Islamist elements in the armed forces.
'Questions remain about who is arming the Fulani and why? Whose agenda do they serve by attacking Christians? And why has the government of Nigeria done so little to disarm them?' said Robinson.
'Nigeria's re-elected president must answer these questions – and take immediate action to stop the violence.'
Release International partner, the Anglican Archbishop of Jos Ben Kwashi, said: 'The government has been unable to provide security for the poor. The government must rise to its responsibility to mete out justice and save lives.'