The U.N. humanitarian chief is accusing Boko Haram of "almost unimaginable" violence and brutality that has forced massive numbers of people to flee their homes and led to unprecedented number of people in need.

Stephen O'Brien said the U.N. estimates that over nine million people across the Lake Chad Basin spanning parts of Nigeria, Niger, Chad and Cameroon need humanitarian assistance, including about 2.8 million who fled violent attacks in their towns and villages.

He told the Security Council that Nigeria is bearing "the brunt of the crisis" despite significant government efforts, with Nigerians accounting for seven million of the nine million people needing help.

U.N. political chief Jeffrey Feltman said that despite regional efforts to weaken Boko Haram, the group continues to threaten regional stability.