Suspected Boko Haram fighters have killed at least 20 members of vigilante groups trying to fight back against the group in north-east Borno state.The deaths occurred in two separate attacks on Sunday and Monday, said residents and a military official.Boko Haram has waged a deadly insurgency in Nigeria since 2009.In May, President Goodluck Jonathan declared an emergency in three north-eastern states, saying the group threatened Nigeria's existence.An offensive was launched against the group - which says it is fighting for the creation of an Islamic state in Nigeria - and the military encouraged the formation of vigilante groups to help.But now it appears Boko Haram is taking revenge against such groups, say observers - adding weight to fears that the vigilante groups may trigger an escalation of the violence.On Sunday, men disguised in military uniforms stormed a meeting of one vigilante group in Bama, opening fire and killing 14, residents said.An official at a local hospital told AFP news agency another four people died on Monday from the attack.The second attack took place on Monday night in the Borno village of Damasak, some 200km (125 miles) away.Attackers crept up on sleeping members of a group, which calls itself the Civilian Joint Task Force, as they slept in a guesthouse and shot them dead, said a relative and a military official speaking anonymously.