NIAMEY - Armed attackers killed 137 people in coordinated raids on villages in southwestern Niger on Sunday, the government said, making it one of the deadliest days in recent memory in a country ravaged by Islamist 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."By systematically targeting civilians, these armed bandits are reaching a new level of horror and savagery," it said in a statement, announcing three days of national mourning. It did not say who authorities believed was behind the attacks. A security source earlier blamed Islamic State, 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 also fueled by militants linked to al Qaeda, Islamic State 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 the cluster of humanitarian organizations led by the U.N. Refugee Agency (UNHCR).Unidentified assailants killed at least 58 villagers in the nearby region of Tillabery last Monday.