ABUJA: A suspected suicide bombers killed 15 civilians in a failed attack on a state oil company facility in northeast Nigeria, when soldiers at a checkpoint opened fire on their explosive-packed vehicles, the military said on Wednesday.Seventeen civilians and five soldiers were wounded by the blasts on Tuesday which also destroyed eight vehicles, Defence Ministry spokesman Chris Olukolade said. There was no immediate claim of responsibility.Olukolade said the bombers, believed to be from militant group Boko Haram, were driving towards the Nigeria National Petroleum Corporation facility at Mule. Four bombers died.Mule is on the outskirts of Maiduguri, the capital of Borno state and the cradle of the insurgency. “Three of the four explosive-laden vehicles were demobilised by shots fired at them by soldiers at the checkpoint, shortly before the explosions that rocked the area,” Olukolade said. “A total of 15 civilians including a member of a youth vigilante group died,” he said.Violence in Borno state is worse than at any time during its 4-1/2-year-old insurgency, residents say. Enraged Boko Haram militants, fighting for an Islamic state in Nigeria, have in the past year broadened their range of targets beyond security forces, government officials and Christians to include school children and other civilians, sometimes massacring whole villages and abducting girls.A military crackdown since last May has failed to rout the insurgency, which remains the leading security threat to Africa’s top oil producer and a serious headache for President Goodluck Jonathan ahead of February 2015 elections.The militants’ use of explosive devices is on the rise again. Suspected Islamists drove a car packed with explosives into a police patrol in Maiduguri a week ago. They also bombed a crowded marketplace on March 27 near the town of Bama.