Nearly a quarter of a million children in Nigeria’s north east are severely malnourished, according to the UN’s Assistant Secretary-General Toby Lanzer.
Millions more are thought to be starving in refugee camps that are too dangerous for aid agencies to reach.
Nigeria is already economically struggling – it is in a recession for the first time in more than a decade, inflation is at an 11-year high and oil, which makes up 70% of the government’s revenue, is still suffering from a slump.
For the country’s north eastern region, however, things are made worse by Islamic militants Boko Haram, who have disrupted trade and farming in the region and are also hostile to western medicine.
Mr Lanzer said that, if Nigeria doesn’t get help fast, “we will see, I think, a famine unlike any we have ever seen anywhere”.
Speaking at a meeting in Brussels, Mr Lanzer said of the millions who haven’t yet been reached: “We can’t assess their situation – we can estimate that it’s awful.”
Jean Stowell is an American midwife in charge of a 110-bed feeding centre in Maiduguri, which is the largest city in the region.
The Doctors Without Borders-run centre tries to keep up with demand but, every time it grows, the extra places are rapidly taken.
Ms Stowell told the Associated Press: “These are kids that basically have been hungry all their lives and some are so far gone that they die here in the first 24 hours.”
There are around a million people in Maiduguri’s camps – most of them fleeing the reach of the terrorist group – and many of them are also struggling with hunger.
And yet, outside the camps, the markets are filled with fruit and vegetables – all of it far too expensive for the average refugee.
The Nigerian government is investigating claims that officials have been stealing food aid.