DAKAR:  Millions of children across the Lake Chad basin are prey to deadly water-borne diseases such as cholera and hepatitis E as the rainy season hits a region already reeling from Boko Haram's insurgency, the United Nations said on Friday.

More than 5.6 million children in Cameroon, Chad, Niger and Nigeria, many of whom have been uprooted by violence and live in host communities or refugee camps, are facing the disease threat as the rains arrive, said the UN children's agency (UNICEF).

Flooding and muddy roads are expected to limit aid access to remote areas, where hunger is growing and food is lacking, while the insecurity has made it hard to deliver supplies and ensure clean water is available ahead of the rains, aid agencies say.

"The rains will further complicate what is already a dire humanitarian situation, as millions of children made vulnerable by conflict are now facing the potential spread of opportunistic diseases," Marie Pierre Poirier of UNICEF said in a statement.

"Unsafe water, inadequate sanitation and poor hygiene conditions can lead to cholera and hepatitis E," said UNICEF's regional director. "Staving off disease is our top priority."

Cholera, which spreads through contaminated food and water, causes diarrhoea and vomiting, leaving small children especially vulnerable to death from dehydration, whereas liver disease hepatitis E is particularly deadly for pregnant women.

In Niger's Diffa region - which has been hit by the conflict and hosts about 250,000 uprooted Nigeriens and Nigerian refugees - an outbreak of hepatitis E has killed at least 33 pregnant women so far this year, said Medecins Sans Frontieres (MSF).

"To curb this type of outbreak, we know that our best asset at the moment is ... water and sanitation activities," said Víctor Illanes of the aid group.

"But when the deficiencies are so high and the space to be covered is as large as Diffa, it is difficult for these activities to have an impact in the short term," he added.

Boko Haram's campaign to create an Islamic state is in its eighth year with little sign of ending. It has claimed more than 20,000 lives and uprooted 2.7 million people across Lake Chad.

More than 5 million people in northeast Nigeria need food aid, and about 1.5 million are believed to be on the brink of famine, yet the United Nations this month had to cut emergency food supplies for 400,000 people due to a lack of funding.