Boko Haram jihadists have laid waste to the impoverished region since taking up arms against the government in 2009, displacing millions and disrupting farming and trade.
Nigeria's president, Muhammadu Buhari, has reclaimed territory from the Islamists but the insurgency has taken a brutal toll, with more than 20,000 people dead, 2.6 million displaced, and famine taking root.
UN humanitarian coordinator Peter Lundberg said the crisis was unfolding at "high speed".
"Our assessment is that 14 million people are identified as in need of humanitarian assistance" by 2017, Lundberg said in the Nigerian capital, Abuja.
Of these, 400,000 children are in critical need of assistance, while 75,000 could die "in the few months ahead of us", Lundberg said.
The UN hopes to target half of the 14 million people - a population bigger than that of Belgium - with the Nigerian government working to reach the rest.
But Lundberg said the UN did not have enough money to avert the crisis and called on international partners, the private sector and Nigerian philanthropists to "join hands" to tackle the problem.
"We need to reach out to the private sector, to the philanthropists in Nigeria," Lundberg said.
"We will ask international partners to step in because we can only solve this situation if we actually join hands."
