(CNN)Flags flew at half-staff and Koranic verses played on state media on Tuesday as Niger began observing three days of national mourning for the 137 victims of coordinated raids on villages in the southwest.
Niger mourns 137 victims after deadliest attack in recent memory
The killings on Sunday represented Niger's worst civilian carnage in recent memory, surpassing an attack by suspected militants in January that killed at least 100 villagers and another last week that killed at least 58.
The scale of the violence this year has called into question claims of progress in the fight against Islamist militants by governments in West Africa's Sahel region and former colonial power France, which has 5,000 troops there to support them.
A security source blamed Sunday's attacks, which were carried out by men on motorbikes in the remote Tahoua region near the border with Mali, on the local Islamic State affiliate. Islamic State has claimed responsibility for previous raids against security forces and French aid workers.
Niger only finished a period of mourning for the previous attacks on Friday.
The rising death toll appears to be the result of the Islamists inserting themselves into ethnic conflicts between rival farming and herding communities, said Harouna Abarchi, a civil society activist who leads peace initiatives in the area.
The militants, largely drawn from the Fulani herding community, have targeted civilians in retaliation for killings of Fulanis by self-defense militias formed by the farmers.
"It's a fragile zone that has seen inter-communal conflicts in the past," Abarchi said. "They are now going after civilians, which ... could set the zone on fire."
The security crisis will land at the feet of President-elect Mohamed Bazoum, who takes office next week, succeeding Mahamadou Issoufou.
Abou Oumarou, a retired colonel and former regional governor, said the repeated attacks raised questions about the military's response.
"How is it that 200 people can move around on motorcycles and no one is aware?" he said. "These forces need to surround these zones so that we know when there is a massive movement."
The government said it is investigating the attacks.