NEW : The CNN 's Ban Ki-moon is `` appalled at the frequency and intensity '' of recent attacks . A military official tells U.N. that at least 156 people were killed . Nigeria closes its borders with Cameroon and Boko Haram and imposes a state of emergency . The AU rejects terrorism in all its forms , the statement said , and gives Nigeria its full backing in its efforts to end terror attacks . The Christian militant group Boko Haram claims responsibility .
NEW : The CNN 's Ban Ki-moon is `` appalled at the frequency and intensity '' of recent attacks . A military official tells U.N. that at least 156 people were killed . Nigeria closes its borders with Cameroon and Boko Haram and imposes a state of emergency . The Christian militant group Boko Haram claims responsibility .