Suspected Boko Haram Islamist militants killed at least 18 villagers who had gone into the bush to collect firewood in Nigeria’s northeastern Borno state, local leaders and security sources said.Other villagers were still missing after the attack on Thursday in Marte Local Government Area, one of the areas hardest hit by a 17-year-old insurgency.Armed groups — some seeking to carve out a Caliphate in the region, others seeking plunder or ransoms and recruits from kidnappings — have killed tens of thousands of people and displaced millions in regular raids.Villagers have come under attack as they stray into rebel areas while pushing into the bush for farming and firewood.The traditional ruler of Marte said 18 bodies had been recovered after the attackers ambushed the villagers.A senior military officer said troops had launched clearance operations in the area and were pursuing the attackers.The insurgency in northeastern Nigeria is led mainly by Boko Haram and its offshoot, Islamic State West Africa Province.