Intensified cross-border attacks by Nigeria's Islamic extremists have created insecurity in the Lake Chad area of West Africa that, if help is not provided, could create a more fertile recruiting ground for Boko Haram, U.N. humanitarian coordinators said Tuesday.

Some 20 million people have been affected by Boko Haram attacks that have spread from northeastern Nigeria to the neighboring countries of Cameroon, Chad and Niger, according to U.N. figures. More than 2.8 million people have been displaced by suicide attacks, village raids and a six-year insurgency that has pledged itself to the Islamic State.

U.N. agencies and non-governmental organizations are appealing for $531 million to reach 5.2 million people across the four countries hit by Boko Haram.

"The sophistication of the group is broader and deeper," said Toby Lanzer, U.N. Assistant Secretary-General and Regional Humanitarian Coordinator for the Sahel.

Boko Haram, which started in Nigeria's northeast, has wreaked the most havoc in Nigeria, where 2.2 million are displaced, but the chaos caused in neighboring countries is growing, said the U.N. officials.

"Its origins are in Nigeria, but it is a transnational organization," said Karl Steinacker, the Niger representative for the U.N. refugee agency. "There is certainly a Niger branch of Boko Haram."

As the multinational military force becomes more effective against Boko Haram, the extremist group is turning more to hostage-taking, banditry and stealing of livestock — threatening the economies and access to food, says U.N. Development Program coordinator of Cameroon, Najat Rochdi. In the last year, she said, displaced people within Cameroon increased from 50,000 to 158,000.

"If we are serious about defeating Boko Haram, not only the military way, but also sustaining and creating the hope for those youth," she said. "We need to invest urgently in early recovery, in empowering the youth, in providing them with the hope."