Nearly half a million children around Lake Chad face "severe acute malnutrition" due to drought and a seven-year violent campaign by the armed group Boko Haram in northeastern Nigeria, UNICEF has warned.

Of the 475,000 deemed at risk, 49,000 in Nigeria's Borno state, Boko Haram's heartland, will die this year if they do not receive treatment, the UN children's agency said on Thursday.

At the start of 2015, Boko Haram occupied an area the size of Belgium but has since been pushed back after Nigeria, Chad, Niger and Cameroon formed a coalition along with Benin to fight the group.

Most of its remaining forces are now hiding inside the vast Sambisa forest, southeast of the Borno provincial capital, Maiduguri.

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UNICEF said that as Nigerian government forces captured and secured territory, aid officials were starting to piece together the scale of the humanitarian disaster left behind in the group's wake.

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"Towns and villages are in ruins and communities have no access to basic services," UNICEF said in a report.

In Borno, nearly two thirds of hospitals and clinics had been partially or completely destroyed and three-quarters of water and sanitation facilities needed to be rehabilitated.

Despite the military gains, UNICEF said, 2.2 million people remain trapped in areas under the control of Boko Haram or are staying in camps, fearful of going home.

Appealing for more funds, UNICEF said that, to date, it had only received $41m - 13 percent of what it needs to help those affected in Chad, Nigeria, Niger and Cameroon that border Lake Chad.

The war between Boko Haram and the Nigerian government has left at least 20,000 people dead in six years and made more than 2.6 million homeless.

Responding to its battlefield setbacks, Boko Haram has turned to suicide bombings, often involving children.

UNICEF said that it had recorded 38 cases of child suicide bombings so far this year, against 44 in the whole of 2015 and only four the year before that.

Droughts and floods have also played a significant role in food insecurity in the region, creating a multi-layered crisis situation for aid agencies to deal with.

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Source: Agencies