Tanks and troops are on patrol in parts of northern Nigeria after the president declared a state of emergency in areas hit by a recent spate of attacks by Islamist militants.

President Goodluck Jonathan imposed the measure Saturday in 15 locations in the northeastern states of Yobe and Borno, Plateau state in central Nigeria, and Niger state in the east.

The state of emergency gives security forces control of those areas. It also closes parts of Nigeria's borders with Niger, Chad and Cameroon.

Mr. Jonathan, in a speech, vowed to 'crush' Boko Haram, the Islamist group blamed for a series of Christmas Day attacks that killed about 40 people, most of them Christians.

The group's name means “Western education is a sin.” It has claimed responsibility for multiple bombings and shootings in the north and in Nigeria's capital, Abuja.

The president said he has directed his chief of defense to take 'appropriate measures' in the affected parts of the country.

In unrelated violence, at least 50 people were killed Saturday in clashes between two rival ethnic communities in southeastern Nigeria. The clashes between the Ezza and Ezilo people in Ebonyi state were most likely over a land dispute.