JOHANNESBURG (AP) — The United Nations says a cholera outbreak in northern Nigeria has killed at least 23 people and threatens thousands of others displaced by the Boko Haram extremist insurgency.

The U.N. humanitarian agency says more than 500 suspected cholera cases have been registered since the first was recorded last month by Nigerian health officials.

Most of the cases have been recorded in the Muna Garage displaced persons camp of about 20,000 people on the outskirts of Maiduguri city, the birthplace of Boko Haram's insurgency. Other cases have been identified in similar camps nearby.

The cholera outbreak in Borno state worsens the vast humanitarian crisis in Nigeria's northeast as millions remain displaced by Boko Haram and Nigeria's government and aid workers struggle to feed them.