A landmine planted by Islamist group Boko Haram killed four Chadian soldiers on patrol near Chad's border with Niger on Saturday, two security sources said. They were traveling in a vehicle that rode over the mine at Kaiga Kindji, in the Lake Chad region, which has been plagued by the militants since 2009. The Nigerian-based Boko Haram wants to create a breakaway Islamic state in the region and once occupied an area the size of Belgium. But a regional offensive led by Nigeria, Chad, Cameroon and Niger - the four countries most affected by Boko Haram - has chased it out of much of that territory.