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, France 24 reports.
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.
