A purported spokesman for Nigerian militant group Boko Haram has warned southerners to leave the country's north, while promising clashes with government soldiers.

Abul Qaqa, who has spoken on behalf of the Islamist group in the past, told journalists in a phone conference late Sunday that the group's fighters will “confront” soldiers sent to parts of four states under a state of emergency declared by President Goodluck Jonathan.

Qaqa said Boko Haram is giving southerners living in northern Nigeria three days to move away, and also called on Muslims living in southern Nigeria to return, saying the group has evidence they will be attacked.

President Jonathan declared the state of emergency in 15 locations Saturday, after months of deadly violence that authorities blame largely on Boko Haram.

The group has claimed responsibility for numerous bombings and shootings across the north and in the capital, Abuja, including a Christmas Day bombing of a Catholic church.

Mr. Jonathan has vowed to crush Boko Haram, which first staged a violent uprising against Nigeria's government in 2009. The group's size and structure remain unclear.

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