U.N. Human Rights chief Navi Pillay has called on Nigerian political and religious leaders to join together in efforts to halt sectarian violence in the country.

In a statement Thursday, Pillay said it is especially important for Muslim and Christian leaders to "condemn all violence," including retaliatory attacks.  She said those efforts could help stop a dangerous situation from "spiraling out of control."

Her comments come a day after the leader of the militant group Boko Haram promised more attacks.

Nigerian authorities blame Boko Haram for hundreds of deaths in bombings and shootings over the past 18 months.  The group has claimed responsibility for several of the attacks, including a Christmas Day bombing of a church near Abuja that killed more than 30 people.

Pillay said if Boko Haram members are judged to have carried out systemic attacks against civilians, then the militants could be found guilty of crimes against humanity.

The group is believed to want wider implementation of sharia, or Islamic law, across Nigeria.  It recently warned Christians in Muslim-majority northern Nigeria to leave the area.

In response to the violence, President Goodluck Jonathan recently declared a state of emergency in 15 locations.

The president has also deployed extra troops to the north, but attacks have continued.