Troops from Niger and Chad discovered a mass grave with more than 90 decomposed bodies near a northern Nigerian town recently retaken from the Islamic militant group Boko Haram, security sources from both countries said Friday.

The corpses, some of them beheaded, were found in a shallow mass grave under a bridge just outside the Nigerian town of Damasak, near the border with Niger, the sources said.

Troops from Niger and Chad took back the town from Boko Haram last week during a joint operation, the security sources said.

Damasak is in Borno, one of several northern states inhabited by the Islamist militant group. Borno is in the northeast corner of Nigeria and borders Niger, Chad and Cameroon.

Boko Haram has terrorized northern Nigeria regularly since 2009, attacking police, schools, churches, mosques and civilians. It has kidnapped students, including more than 200 schoolgirls who were abducted in April 2014 and remain missing.

The group, its naming meaning "Western education is forbidden," has said its aim is to impose a stricter form of Sharia law across Nigeria.