Nigerian Islamists kill at least 159 in two attacks BENISHEIK, Nigeria (Reuters) - Islamist Boko Haram militants killed 159 people in two roadside attacks in northeast Nigeria this week, officials said, far more than was originally reported and a sign that a four-month-old army offensive has yet to stabilize the region. In the first attack, on Tuesday, Boko Haram guerrillas wearing army uniforms stopped traffic on a highway between the cities of Maiduguri and Damaturu, dragging people out of their vehicles and killing them, with 143 bodies recovered so far. Violence in northeast Nigeria has intensified over the past two months, as the Islamists fight back against a military operation that President Goodluck Jonathan ordered in May to try to crush their four-year-old rebellion. Tuesday's toll was initially given as "more than 20," but information often takes days to trickle out of the remote and sparsely populated region, where roads are bad, curfews are in force and the military has cut the phone network since May. "We have been picking corpses off the roadsides all day, there are more in the bush," said Abdulazeez Kolomi, an Environmental Protection Agency official in Benisheik village. They are all travelers slaughtered by Boko Haram gunmen. We have so far picked up 143 corpses. On Thursday, following a similar pattern, Boko Haram insurgents killed at least 16 people in an attack on travelers plying a highway from Maiduguri to Bamboa, a police source collecting bodies on the scene told Reuters. Thousands have been killed since the shadowy sect launched its uprising against the state in 2009, turning itself from a clerical movement opposed to Western culture into an armed militia with growing links to al Qaeda's West African wing. The military operation that started in May brought an initial lull to the violence, as the Islamists fled their bases in cities, forests and mountains across the northeast. But the lull quickly gave way to revenge. First schools were targeted, as the Islamists thought they were hiding vigilantes. Then attacks on security forces resumed. The orgy of violence over the past few weeks - several hundred people have been killed - suggests the offensive against Boko Haram has not worked and may have made things worse. "They have taken to guerrilla tactics in rural areas, where the population are vulnerable," said Kole Shettima, Africa director of the MacArthur Foundation, based in Nigeria. The military were winning some battles, but military deployments cannot win the war. Boko Haram can simply adapt," he said, adding that a broader strategy including investment in the underdeveloped north and some kind of political solution was needed. Boko Haram, whose name means "Western education is sinful" in the northern Hausa language, wants to revive the medieval Islamic kingdoms that used to rule northern Nigeria, before its amalgamation with the largely Christian south by the British colonial authorities. They are seen as the gravest security threat to Africa's top oil producer. Although their activities are located hundreds of miles away from its southern oil fields, they have bombed the capital Abuja at least three times, including a deadly attack on the United Nations' Nigeria headquarters in 2011. Nigeria's military said on Wednesday it had killed 150 insurgents, including a commander, in an operation against Islamist group Boko Haram in which 16 of its own forces were also killed. The Nigerian army often says it killed large numbers of insurgents in battles in which a much smaller number of its own troops died. It is impossible to independently verify this. Violence in the northeast is unwelcome news for Jonathan, who is suffering a split in his party over an assumed intention to run again in 2015 elections, and from an opposition coalition that seems better organized and funded than in the past. He had been criticized for not quelling Boko Haram's insurgency, which worsened under his leadership. The state of emergency he declared in May was seen as a last ditch attempt to show he is on top of the crisis. Reporting by Lanre Ola; Writing by Tim Cocks; Editing by Kevin Liffey and Sonya Hepinstall