Extremist group Boko Haram claimed an attack that killed 28 people and at burnt 800 homes in Niger’s Diffa region on Saturday, local authorities and the UN said on Monday.
The attack took place in the village of Toumour, about 19 kilometres from the Nigerian border, the government.
About 60,000 internally displaced persons and refugees lived in the area.
Attackers arrived on foot in the village on Saturday evening, after swimming across Lake Chad, an official said.
The incident took place just days before the region’s elections.
Boko Haram claimed responsibility for the attack in a three-minute video sent to AFP.
The terrorist group threatened Christians that there would be more attacks before Christmas.
Government spokesman Abdourahamane Zakaria on Monday declared a three-day period of national mourning.
Mr Zakaria said that 10 people died from gunshots, 14 by fire, and four drowned. About 100 people were wounded.
Boko Haram has been waging attacks in the region around Lake Chad since 2009, causing about 250,000 to flee, the UN says.
The insurgency erupted in north-eastern Nigeria but violence frequently spills over into neighbouring Chad, Niger and Cameroon a little farther to the south.
The bloodshed in Toumour is among the worst the country has suffered at the hands of the militants, Diffa Governor Issa Lemine said on Sunday.
"It is an indescribable tragedy," Mr Lemine said, as he recalled the panic after the attack.
"It was an attack of unprecedented savagery," an elected local official, told Radio France Internationale.
"Nearly 60 per cent of the village has been destroyed."
A study released last week found that seven African nations are among the 10 most dangerous global locations for terrorism.
Attacks by groups such as Boko Haram and IS West Africa Province are on the rise.
Terrorist incidents in Africa rose by 13 per cent in the past quarter, research by Verisk Maplecroft showed.