Southern Kordofan is important to the north because it has the most productive oil fields that will remain under Khartoum's control after the split. The south could take as much as 75 percent of Sudan's 500,000 barrels per day of oil output.
It also borders Abyei and Darfur, a western region that is the scene of a separate insurgency.
Analysts have predicted fighting could break out in Southern Kordofan ahead of the split, especially after an NCP official was named the winner of a gubernatorial election last month. The south said the vote was rigged, which Khartoum denied.
Officials with the south's dominant party, the Sudan Peoples' Liberation Movement, have said the clashes started when the north tried to disarm armed groups in the area.
The northern army has blamed the southern-aligned groups for starting the fighting.
The militias are still referred to as members of the Southern Peoples' Liberation Army -- the southern military -- although Juba says they are no longer part of their army and cannot ask them to withdraw south because they are northern.
Southern Sudan voted to secede in a January referendum, the culmination of a 2005 peace deal that ended decades of civil war between the north and south.