JUBA, Sudan — Northern and southern Sudanese officials have agreed to a preliminary arrangement on demilitarizing the border between them, the African Union said Tuesday, though some officials from both sides immediately expressed skepticism of the deal, particularly its ability to resolve the dispute over the contested Abyei area.

Sudan’s internal north-south border — which will soon mark an international boundary between northern Sudan and the soon-to-be-independent nation of South Sudan — has been the site of intense clashes in recent months. Both north and south have mustered a large number of forces along the divide, and on May 21 the northern Sudanese army and local militiamen seized the Abyei area, setting off fears of a larger border war.

But according to a deal reached on Monday night and disclosed by African Union officials on Tuesday, both sides have agreed to establish a demilitarized 12.4-mile-wide zone along the roughly 1,240-mile border, and to have a high-level body with representatives from each side to monitor security arrangements that affect Sudan, both north and south, though its workings are still vague.

“The agreement paves the way for further negotiations on key security issues between the parties, to be convened by the A.U. Panel within the week,” an African Union statement said.

Still, there was ample skepticism. Both sides have broken treaties before.

“The question is, whether the Khartoum army and intelligence implement it?” said Col. Philip Aguer, a spokesman for the southern military. “We doubt it very much.”

Rabie A. Atti, a spokesman for the northern government in Khartoum, said the agreement could help foster “peaceful coexistence between both parties.” But when asked if this meant the northern army would withdraw from Abyei and the rest of the border areas, he said, emphatically, “No, no, no.”

Western officials have said that there will be no durable peace in Sudan until Abyei is resolved, and that the leading proposal was to send in thousands of Ethiopian soldiers to serve as a buffer between northern and southern forces until a final settlement was hammered out. Abyei, which has an abundance of fertile land and some oil, lies right on the north-south border.

Sudan is increasingly tense as the south prepares to declare independence, scheduled for July 9. Thousands of northern soldiers have been deployed in two other areas along the border, Blue Nile and Southern Kordofan, and have demanded that all southern-allied soldiers in those areas — and there are thousands — disarm by Wednesday.

It remains unclear whether the north intends to carry out a forcible disarmament of southern-allied forces, or whether it will heed the advice of Western and African officials to pause and give negotiations a chance to defuse tensions.

On Tuesday night, a southern-aligned official in Kadugli, the capital of Southern Kordofan, said large numbers of northern soldiers and tanks had moved into the capital over the last 24 hours. Tensions were growing, but clashes had not yet broken out, he said.

“We are expecting these things to happen either tomorrow or the day after tomorrow,” said Attia Atroun Attia. “It could happen anytime.”