US President Donald Trump has ordered the military to prepare for action in Nigeria to tackle Islamist militant groups, accusing the government of failing to protect Christians.

Trump did not say which killings he was referring to, but claims of a genocide against Nigeria's Christians have been circulating in recent weeks and months in some right-wing US circles.

Groups monitoring violence say there is no evidence to suggest that Christians are being killed more than Muslims in Nigeria, which is roughly evenly divided between followers of the two religions.