Troops from Northern Sudan have moved into Abyei - a town on the border with Southern Sudan.
South Sudan is due to become independent in July, but Abyei is claimed by both sides.
The BBC reports the dispute is seen as a possible trigger for a new war.
State television in Khartoum said northern troops had ''repelled enemy forces'' in Abyei. UN officials have confirmed the development.
A delegation from the United Nations is due to visit on Monday.
The BBC reports there have been clashes between northern and southern forces in the area since Thursday.
The UN said the northern troops were being escorted out of Abyei by UN peacekeepers.
The US called on South Sudan to ''account'' for the assault.
Washington said the attack was ''in direct violation'' of an agreement signed by the north and south in January to "remove all unauthorised forces" from Abyei.
South Sudanese forces denied responsibility for the incident.