"A coalition of terrorist armed groups" attacked security forces near the village of Tassia, leaving "21 martyrs including one civilian" and nine injured, the ministry announced in a statement on national television.

It said "several dozen" of the assailants were killed and that aerial and ground reinforcements were being deployed to track down the rest of the attackers.

Three days of national mourning will begin from Wednesday with flags lowered to half-mast, the ministry said, speaking of the security forces' "unshakeable determination" to "continue this fight for sovereignty".

Tassia lies in the Tillaberi region bordering Mali and Burkina Faso where rebels linked to Al-Qaeda and the Islamic State group have waged a bloody insurgency for almost a decade.