The U.S. says it is troubled by clashes between northern and southern Sudanese forces ahead of Sudan's split next month into two separate nations.
Secretary of State Hillary Rodham Clinton will raise U.S. concerns when she travels to Africa later this week.
State Department spokesman Mark Toner said Tuesday the fighting in the Southern Kordofan and Blue Nile border states must cease immediately.
These states are northern territory but many inhabitants fought for the south in their two-decade civil war.
The standoff follows the north's occupation last month of the disputed Abyei region. The U.N. says 100,000 fled the fighting.
Toner said border and security issues should be resolved peacefully.
Most of the south's oil is in states bordering the north. Those areas saw brutal conflict during the war.