Nearly half a million children face starvation in north-eastern Nigeria and 80,000 will die unless they receive treatment, the United Nations' children's agency has warned.

The region is in the grip of a "catastrophic" humanitarian crisis created by Boko Haram's Islamic uprising that has killed more than 20,000 people and left over two million displaced.

The UN said they are "extremely concerned" for the 400,000 children trapped in the Borno state left largely inaccessible to aid agencies by the on-going seven-year uprising.

"If they do not receive the treatment they need, one in five of these children will die," Unicef executive director Anthony Lake said.

Fears for children in Maiduguri, the biggest city in Nigeria's north east, were raised earlier this year.

Doctors Without Borders said in November that thousands of children already have died, including 10 to 25% of children admitted to its 110-bed Maiduguri emergency treatment centre.

Nigeria's senate is investigating allegations that government agencies are diverting food aid that could help prevent those deaths.