A number of inmates escaped from a prison holding militants in Niger’s western Tillaberi region on Thursday, the country’s interior ministry said in a statement.

The ministry did not say how many inmates escaped or how they broke out of the Kotokali prison, northwest of the capital Niamey and considered the most heavily guarded in the country.

The prison holds many members of Nigeria’s Boko Haram and other militant groups in the Sahel.

Local authorities imposed a night-time curfew in the urban area of Tillaberi, where the prison is located, without giving further details.