Scores of people have died in the deadliest suspected jihadist massacre ever to hit Niger, the government said Monday, underscoring the huge security challenge facing new President Mohamed Bazoum. 

Government spokesman Zakaria Abdourahamane said 137 people had 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," Abdourahamane said in a statement read on public television.

Announcing three days of national mourning for the victims, he vowed that the government would reinforce security in the region and bring "the perpetrators of these cowardly and criminal acts" to justice.

Gunmen arriving on motorbikes attacked the villages of Intazayene, Bakorat and Wistane on Sunday, shooting "at everything which moved," a local official said.

The jump in the death toll, which had been given as at least 60 earlier Monday, would make Sunday's attacks the deadliest ever committed by suspected jihadists in Niger. It brings the number of fatalities in the Mali-Niger border region to 236 in just over a week.

The world's poorest nation according to the UN's development rankings for 189 countries, Niger is also struggling with Islamist insurgencies that have spilled over from Mali and Nigeria.

Hundreds of lives have been lost, nearly half a million people have fled their homes, and devastating damage has been inflicted in the former French colony.   Niger is part of a France-backed alliance of countries in the Sahel region known as the G5. A contingent of 1,200 troops from the Chadian army, considered the region's toughest, has been deployed under the G5 banner.