A purported spokesman for Nigerian Islamist group Boko Haram has threatened to confront soldiers after the president declared an emergency in areas hit by attacks blamed on the militants.

"We find it pertinent to state that soldiers will only kill innocent Muslims in the local government areas where the state of emergency was declared," Abul Qaqa told journalists in a phone conference late Sunday.

"We would confront them squarely to protect our brothers."

Qaqa, who has spoken on behalf of the group numerous times in the past, also threatened southern Nigerians living in the north of the country, roughly divided between a mainly Muslim north and predominately Christian south.

"We also wish to call on our fellow Muslims to come back to the north because we have evidence that they would be attacked," he said, speaking in the Hausa language common throughout the north.

"We are also giving a three-day ultimatum to the southerners living in the northern part of Nigeria to move away."

President Goodluck Jonathan declared a state of emergency on Saturday in parts of four states hard hit by violence blamed on Boko Haram.

The move came in response to scores of attacks attributed to Boko Haram, particularly a wave of bombings on Christmas that killed 49 people, most in an attack at a Catholic church in the northeast of the country.