The United Nations Mission in Sudan says new clashes between northern and southern forces have killed six people.

U.N. officials said Tuesday the latest violence took place once again in Kadugli, the capital of South Kordofan state.

The United States said it was deeply troubled by reports of the latest violence. A U.S. State Department spokesman called on Sudan's leaders to resolve outstanding issues between them peacefully and quickly and to allow the U.N. mission in Sudan full access to protect civilians.

Earlier Tuesday, the United Nations refugee agency estimated that about 100,000 people have fled unrest in the disputed Abyei region on the north-south border.

U.N. officials believe many other displaced persons are still hiding in the bush and desperately need food, water and other basic aid.

Many of the refugees began fleeing the north-south border region last month, after northern troops seized control of Abyei.

The south is preparing to declare independence from the north on July 9, and the two sides have not agreed on the future of the oil-rich region.

Khartoum has rejected calls by the U.N. Security Council to withdraw its forces.

Renewed tensions between the north and south have raised fears that Sudan may be on the brink of another civil war.

North and south Sudan fought for 21 years before the conflict ended with a 2005 peace agreement.