Armed attackers killed 137 people in co-ordinated raids on villages in south-western Niger on Sunday, the government said, making it one of the deadliest days in recent memory in a country ravaged by violence.
The unidentified assailants struck in the afternoon, raiding three villages and other hamlets in Tahoua region bordering Mali, the government said on Monday, revising the toll up from a previous estimate by local authorities of about 60 killed.
Government spokesman Zakaria Abdourahamane said 137 people died in Sunday's raids in villages near Niger's border with Mali.
"In treating civilian populations systematically as targets now, these armed bandits have gone a step further into horror and brutality," Mr Abdourahamane said.
Announcing three days of national mourning for the victims from Tuesday, he said the government would reinforce security in the region and bring "the perpetrators of these cowardly and criminal acts" to justice.
Mr Abdourahamane did not say who authorities believed was behind the attacks. A security source earlier blamed ISIS, whose local affiliate is active in the zone.
The violence is part of a wider security crisis across West Africa's Sahel region, which is fuelled by militants linked to Al Qaeda, ISIS and ethnic militias.
The attacks on Sunday might have been revenge for the recent arrests of people suspected of belonging to armed groups in the area, said a report by humanitarian organisations led by the UN Refugee Agency.
Unidentified assailants killed at least 58 villagers in the nearby region of Tillabery last Monday.