In its short, seven-year insurgency, Boko Haram has emerged as one of the most ruthless and deadly radical militant groups on the planet.

But the 20,000 people killed in the recent clashes in northeastern Nigeria could be dwarfed by the number of children facing death through malnutrition in areas affected by the fighting, the UN has warned.

Nowhere else in the world is suffering a humanitarian crisis on the same scale, according to Unicef's chief of nutrition in Nigeria, Arjan de Wagt.

The UN Children's Fund estimates as many as 75,000 children will die in the coming year if donors do not act quickly to address the famine-like conditions in the area.

Of 4 million people in desperate need of food are about 2.2 million people trapped in areas where Boko Haram is operating or in newly liberated areas that still are too dangerous to reach by road, de Wagt said.

Most severely malnourished children die of secondary illnesses like diarrhea and respiratory infections, de Wagt told The Associated Press. "But with famine, you actually die of hunger," and that is what is happening, he said.

Severe malnutrition is being found in 20, 30 and even 50 per cent of children in pockets of the region, he said.

"Globally, you just don't see this. You have to go back to places like Somalia five years ago to see these kinds of levels," de Wagt said. Nearly 260,000 people died in Somalia between 2010 and 2012 from severe drought aggravated by war. At the time, the United Nations said aid needed to be provided more quickly.

Unicef on Thursday doubled the amount of its appeal for Nigeria, saying $115 million (£88 million) is needed to save children whose "lives are literally hanging by a thread." Only $24 million has been raised so far, the agency said.

The lack of money has meant some 750,000 people living in accessible areas could not be helped this year, spokeswoman Doune Porter said.

The crisis has reached "catastrophic levels" for people who have sought refuge in towns controlled by the military but who are "entirely reliant on outside aid that does not reach them," aid group Médecins sans frontières said.

And Oxfam's aid group spokeswoman, Christina Corbett, said: "Many families are only able to eat once every few days and usually only watered-down porridge. They are going to bed hungry and waking up with no way to change that."

The rise of Boko Haram Show all 20 1 /20 The rise of Boko Haram The rise of Boko Haram Boko Haram The leader of the Islamist extremist group Boko Haram Abubakar Shekau delivers a message. Boko Haram has claimed responsibility for the mass killings in the north-east Nigerian town of Baga in a video where he warned the massacre “was just the tip of the iceberg”. As many as 2,000 civilians were killed and 3,700 homes and business were destroyed in the 3 January 2015 attack on the town near Nigeria's border with Cameroon AFP The rise of Boko Haram Boko Haram People displaced as a result of Boko Haram attacks in the northeast region of Nigeria, are seen near their tents at a faith-based camp for internally displaced people (IDP) in Yola, Adamawa State. Boko Haram says it is building an Islamic state that will revive the glory days of northern Nigeria's medieval Muslim empires, but for those in its territory life is a litany of killings, kidnappings, hunger and economic collapse The rise of Boko Haram Boko Haram Nitsch Eberhard Robert, a German citizen abducted and held hostage by suspected Boko Haram militants, is seen as he arrives at the Yaounde Nsimalen International airport after his release in Yaounde, Cameroon on 21 January 2015 The rise of Boko Haram Boko Haram Officials of the Nigerian National Emergency Management Agency (NEMA) visit victims of a bomb blast in Gombe at the Specialist Hospital in Gombe. According to local reports at least six people were killed and 11 wounded after a bomb blast in a marketplace in Nigeria's northeastern state of Gombe on 16 January 2015. Islamist militant group Boko Haram has been blamed for a string of recent attacks in the North East of Nigeria The rise of Boko Haram Boko Haram People gather at the site of a bomb explosion in a area know to be targeted by the militant group Boko Haram in Kano on 28 November 2014 The rise of Boko Haram Boko Haram People gather to look at a burnt vehicle following a bomb explosion that rocked the busiest roundabout near the crowded Market in Maiduguri, Borno State on 1 July 2014. A truck exploded in a huge fireball killing at least 15 people in the northeast Nigerian city of Maiduguri, the city repeatedly hit by Boko Haram Islamists The rise of Boko Haram Boko Haram President Goodluck Jonathan visits Nigerian Army soldiers fighting Boko Haram Getty Images The rise of Boko Haram Boko Haram Displaced people from Baga listen to Goodluck Jonathan after the Boko Haram killings AFP/Getty The rise of Boko Haram Boko Haram Nigeria's President Goodluck Jonathan speaking to troops during a visit to Maiduguri, the capital of Borno State; most of the region has been overrun by Boko Haram AFP/Getty The rise of Boko Haram Boko Haram Members of the Nigerian military patrolling in Maiduguri, North East Nigeria, close to the scene of attacks by Boko Haram EPA The rise of Boko Haram Boko Haram Boko Haram’s leader, Abubakar Shekau, appears in a video in which he warns Cameroon it faces the same fate as Nigeria AFP The rise of Boko Haram Boko Haram Nana Shettima, the wife of Borno Governor, Kashim Shettima (C) weeps as she speaks with school girls from the government secondary school Chibok that were kidnapped by the Islamic extremist group Boko Haram, and later escaped in Chibok The rise of Boko Haram Boko Haram South Africans protest in solidarity against the abduction of hundreds of schoolgirls in Nigeria by the Muslim extremist group Boko Haram and what protesters said was the failure of the Nigerian government and international community to rescue them, during a march to the Nigerian Consulate in Johannesburg The rise of Boko Haram Boko Haram Boko Haram militants have seized the town in north-eastern Nigeria that nearly 300 schoolgirls were kidnapped from in April 2014 AFP The rise of Boko Haram Boko Haram A soldier stands guard in front of burnt buses after an attack in Abuja. Twin blasts at a bus station packed with morning commuters on the outskirts of Nigeria's capital killed dozens of people, in what appeared to be the latest attack by Boko Haram Islamists, April 2014 The rise of Boko Haram Boko Haram The aftermath of the attack, when Boko Haram fighters in trucks painted in military colours killed 51 people in Konduga in February 2014 AFP/Getty Images The rise of Boko Haram Boko Haram The leader of Boko Haram, Abubakar Shekau (with papers) in a video grab taken in July 2014 AFP/Getty The rise of Boko Haram Boko Haram Ruins of burnt out houses in the north-eastern settlement of Baga, pictured after Boko Haram attacks in 2013 AP The rise of Boko Haram Boko Haram A Boko Haram attack in Nigeria, 2013 AFP/Getty Images The rise of Boko Haram Boko Haram Abubakar Shekau, Boko Haram’s leader AP

The Associated Press has previously reported recent allegations by displaced people and aid workers that food aid is being stolen in Maiduguri. Nigeria's government has said it would investigate.

MSF said Nigerian authorities are responsible for ensuring aid is delivered and described the overall aid response as "massively insufficient, uncoordinated and ill-adapted."

Muhammad Kanar, the area coordinator for Nigeria's National Emergency Management Agency, denied there is even one case of malnutrition in Maiduguri. Some officials from his agency, which manages the camps, are among several accused of stealing food aid.