Boko Haram jihadists have laid waste to the country's northeast since taking up arms against the government in 2009, displacing millions.
UN humanitarian coordinator Peter Lundberg said the crisis was unfolding at "high speed."
"Currently our assessment is that 14 million people are identified as in need of humanitarian assistance," Lundberg told reporters in the Nigerian capital Abuja.
Out of the 14 million people who need aid, 400,000 children are in critical need of assistance, while 75,000 could die "in (the) few months ahead of us," Lundberg said.
Nigerian President Muhammadu Buhari has reclaimed territory from the Islamists but the insurgency has taken a brutal toll, with over 20,000 people dead, 2.6 million displaced from their homes, and famine taking root.