More than 500 people died from cholera in the Lake Chad region since the start of the year, making it the worst outbreak to hit the area in four years, the UN said on Wednesday.
"Over 27,000 cholera cases and 510 deaths have been reported as of week 35 in the Lake Chad Basin," the UN's Office for the Co-ordination of Humanitarian Affairs said in a report.
"This is 10 times more than the average cholera caseload over the past four years," it said.
It said that without action to control the disease, more than six million people could be affected.
"The region is facing floods and heavy rains, creating an ideal environment for the outbreak to spread," the OCHA warned. "The last two major outbreaks in the region were in 2010 and 2014. An outbreak in 2018 is consistent with the four-year cycle."
The Lake Chad region straddles parts of Nigeria, Cameroon, Chad and Niger, which are also grappling with an extremist insurgency. The OCHA said Nigeria was the worst hit, with 24,000 cases.
Cholera is caused by a bacterium transmitted through contaminated food or drinking water. It causes acute diarrhoea, with children particularly at risk.
Water-borne diseases are a constant threat in the Lake Chad region because of a lack of sanitation. Stagnant groundwater during the rainy season can also cause illnesses.