Nearly half a million children around Lake Chad face "severe acute malnutrition" due to drought and a seven-year insurgency by Islamist militant group Boko Haram in northeastern Nigeria, UNICEF said on Thursday.

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, according to the United Nations' child agency, which is appealing for $308 million to cope with the crisis.

However, to date, UNICEF said it had only received $41 million, 13 percent of what it needs to help those affected in the four countries - Chad, Nigeria, Niger and Cameroon - that border Lake Chad.

At the start of 2015, Boko Haram occupied an area the size of Belgium but has since been pushed back over the last 18 months by military assaults by the four countries.

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

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. "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 - which is trying to establish a caliphate in the southern reaches of the Sahara - or are staying in camps, fearful of going home.

Boko Haram is thought to have killed as many as 15,000 people since the launch of its insurgency in 2009.

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

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