Sudan has declared a state of emergency in several parts of the country bordering South Sudan.
According to the Suna news agency, the decree will apply in the border districts of the South Kordofan, White Nile and Sennar states.
Sudan says more than 12,000 South Sudanese citizens will have to leave the country within a week.
They are part of an estimated 350,000 southerners who moved to the north soon after the 2002 peace agreement looking for work.
South Sudan recently occupied the Heglig oilfield area for 10 days and the BBC reports that various clashes along the border have sparked fears there could be a full-scale war.
South Sudan became independent from Sudan last year after a civil war that lasted two decades. An estimated 1.5 million people were killed.