Tens of thousands have fled across the border from Nigeria to Niger and Benin to escape violence and insecurity this year, the United Nations has said.

Northern Nigeria faces rising religious intolerance and violence by jihadists and criminal bandit gangs, as well as farmer-herder clashes.

The UN's refugee agency (UNHCR) said in a statement on Thursday that more than 66,000 had fled across the borders by mid-October, three times the number in the same period last year.

The agency is struggling to get help to those in need and has appealed for $2 million not only to help those who have fled but also tens of thousands still in Nigeria.

UNHCR spokesperson in Geneva Eujin Byun said relief efforts were being further hampered as 34,000 people had not yet registered, "leaving them at heightened risk and struggling to access essential assistance".

Jihadist violence in northern Nigeria has spilled across into Niger, Chad and Cameroon, prompting a regional military coalition to fight the militant groups.