The death toll from a cholera outbreak at displaced persons camps in Nigeria's northeastern Borno state has risen to 28, the United Nations Children's Fund (UNICEF) said.

The number of those infected at the camps, which house people displaced by the war against the Boko Haram terrorist group has risen to 837, including 145 children under five years old, the agency said in a statement.

"There is growing concern for the health and wellbeing of 1.4 million displaced people, including 350,000 children under the age of five, living in cholera ‘hotspots' in northeast Nigeria," said UNICEF.

The outbreak comes amid ongoing violence and military efforts against the insurgents in the region that has displaced more than 1.7 million people and left over 3.6 million without adequate access to basic water services.

The cholera outbreak in Borno state worsens the vast humanitarian crisis in Nigeria's northeast as millions remain displaced by Boko Haram and Nigeria's government and aid workers struggle to feed them.

UNICEF said the outbreak was first identified in the Muna Garage camp for the displaced in Maiduguri, the state capital of Borno state, but has now spread to as many as six other locations in Borno.