Nigeria and Cameroon have signed an agreement with the United Nations High Commission for Refugees to start “voluntarily and in a dignified manner” the return of some 80,000 Nigerians who sought refuge in the neighboring country, the National Emergency Management Agency said in a statement on Sunday.

Attacks by the Islamist militant group Boko Haram have forced about 2.2 million people to flee their homes, according to the African Union.

The group has killed thousands during a seven-year campaign to establish Islamic law in Africa’s biggest economy and the violence has spilled over beyond Nigeria to Cameroon, Chad and Niger.