At least 87 people have been killed in an attack by Boko Haram militants in Nigeria's north-eastern Borno state, according to local officials.Disguised in military uniforms, the militants set up checkpoints outside the town of Benisheik and shot dead those trying to flee, witnesses said.They also razed dozens of buildings in Tuesday's attack.Boko Haram, which wants to create an Islamic state across Nigeria, has waged a deadly insurgency since 2009.Communications with Borno state have been severely disrupted since May, when a state of emergency was declared in Borno and two neighbouring states.But attacks have increased recently despite a massive military deployment to worst-affected areas.Local vigilante groups have been formed to help counter the militants but scores of these volunteers have been killed in recent weeks.Borno state governor Kashim Shettima visited the scene on Thursday, and described the killings as "barbaric and un-Islamic", and pledged financial assistance to relatives of the victims.The Boko Haram members drove into the town in about 20 pick-up trucks, AFP quoted an anonymous security source as sayingHe said they were heavily-armed, some with "anti-aircraft guns".Benisheik was also the scene of fierce clashes earlier this month, which reportedly left five militants and 13 vigilante members dead.The town lies 70km (45 miles) west of the state capital, Maiduguri, where Boko Haram was founded in 2002.Last month, the army said it had killed Boko Haram's leader Abubakar Shekau but this has not been confirmed and the militants' attacks have continued.Observers say it appears Boko Haram is taking revenge against vigilante groups, adding weight to fears that the strategy may trigger an escalation of the violence.