Army commander in eastern Congo killed in ambush KINSHASA (Reuters) - The colonel in charge of military operations against rebels in eastern Democratic Republic of Congo was killed on Thursday in an ambush along with three other soldiers, government spokesman Lambert Mende said. Colonel Mamadou Ndala, who helped to secure the military defeat of the M23 rebel group in November, died of his injuries after a rocket attack on his vehicle near the village of Mazizi in North Kivu province, Mende said. His killing indicated the challenge facing the Congolese government and a 21,000-strong U.N. peacekeeping mission in its efforts to pacify dozens of armed groups operating in the east of the vast central African nation. "Colonel Mamadou Ndala has just passed away," Mende told Reuters by telephone, adding that the three other soldiers in the vehicle were also dead. We have not yet identified the attackers. Ndala was in charge of tackling the ADF-NALU rebels, a Ugandan group which has stepped up its attacks in eastern Congo this year and has been identified by a U.N. peacekeeping mission as one of the main obstacles to peace in the mineral-rich region. ADF-NALU has been blamed for a spate of recent attacks and kidnappings around the town of Beni in North Kivu, including the deaths of at least 60 civilians in two attacks last month. Ndala was returning from Beni airport without a military escort, sources said. Reporting by Bienvenu Bakumanya; Writing by Daniel Flynn; Editing by Ruth Pitchford