The United Nations Mission in Sudan says Sudanese troops have arrested six staff members who were being evacuated from a volatile north-south border state.

The mission said the staffers, all of whom are Sudanese nationals, were detained at the airport in Southern Kordofan state on Wednesday.

Spokesman Kouider Zerrouk condemned the arrests, saying they violate agreements that guarantee freedom of movement for U.N. staff and called for the staff members to be released.

Violence has been escalating along the north-south border as southern Sudan prepares to declare independence on July 9.

U.S. President Barack Obama Wednesday described the situation in Southern Kordofan state as “dire” and pressed the north and south to immediately impose a cease-fire.

Mr. Obama said there are reports that attacks are taking place in Southern Kordofan state based on ethnicity. He accused the north's Sudanese Armed Forces of bombing civilians and harassing and intimidating United Nations peacekeepers in the region.

He warned that if a cease-fire is not implemented, the Sudanese government will only face more international isolation.

Mr. Obama also urged both northern forces and troops with the south's Sudan People's Liberation Army to allow humanitarian access to civilians who desperately need help.

The United Nation's says more than 70,000 people have fled fighting in Southern Kordofan. U.N. officials say the whereabouts and conditions of many of them are unknown.

Earlier this week, negotiators reached an agreement on Sudan's disputed Abyei region, where north-south fighting had also broken out.

The north and south agreed to a complete demilitarization of the oil-rich region and the deployment of U.N. peacekeepers from Ethiopia. They also agreed to set up a council to oversee security in Abyei.

President Obama praised the Abyei agreement and expressed hope that a cease-fire in Southern Kordofan will allow peace negotiations to move forward.