BAMAKO—At least 51 people were killed when Islamist militants raided three villages in central Mali near the border with Niger, a district administrator said on Monday. Ouatagouna, Karou, and Deouteguef were simultaneously attacked around 6 p.m. on Sunday, according to a note from the Asongo district administrator to the governor of Gao region. The United Nations mission in Mali said it would deploy additional troops to secure areas where the attacks took place and intensify existing patrols. Houses were ransacked and burned to the ground and herds of livestock carried away, said the administrator’s note, which was seen by Reuters. “Provisional toll is 51 killed, several other injured,” it said. No group has yet taken responsibility for the attacks in the area where Malian troops, French and European forces, and U.N. peacekeepers have been battling insurgents linked to the ISIS and al-Qaeda terrorist groups. “These deliberate attacks against civilian populations …