He said that Maiduguri, a city in northeastern Nigeria, has seen its population swell to 2.6 million following an influx of 1.6 million people fleeing violence.

In total, 2.6 million have been forced from their homes by the fighting, 2.2 million of whom are Nigerians, Lanzer said. He made his comments following a tour of the Lake Chad basin region, which touches all four affected countries.

The U.N. envoy stressed that while refugees fleeing Boko Haram violence were not currently a significant contributor to Europe's migrant crisis, that could change if the situation around Lake Chad remains dire.

"Is that something that could be on the cards? Absolutely," Lanzer told journalists, warning that with a rising population, crippling poverty and continuing violence, significant numbers of young men around Lake Chad could increasingly try to migrate towards Europe.

Nigeria's President Muhammadu Buhari, who came to power in May, has instructed his military commanders to crush the insurgency by the end of the year.

Lanzer offered broad praise for Buhari's response to the crisis.

"There is a pretty realistic appreciation of the gravity of the situation," he said, specifying that he was speaking specifically about the administration led by Buhari, who took power from Goodluck Jonathan, whose handling of the conflict was strongly criticized.

Al Jazeera and Agence France-Presse