The northern Sudanese Army says it has taken control of Abyei, a contested area on the border with South Sudan.
Sudanese state television, based in Khartoum, said northern troops had "repelled enemy forces" in Abyei town. UN officials confirmed the development.
The move follows three days of clashes between northern and southern forces in the area.
The dispute over the oil-rich region is seen as a possible trigger for a new north-south civil war.
South Sudan is due to become independent in July, but Abyei is still claimed by both sides.
The BBC's James Copnall in Khartoum says the key question is whether the fighting is now over or whether a new civil war is about to begin.
"The SAF (Sudan Armed Forces) have entered Abyei," said Philip Aguer, spokesman for the southern Sudan People's Liberation Army (SPLA) rebels. "There is still fighting but they have come with tanks, they are shooting and shooting."
A UN official told AFP news agency: "The SAF have taken the town, it is a major development."
A UN Security Council delegation is currently in Sudan and is due to visit Abyei on Monday.
Our correspondent says it now faces an even more worrying situation than it could have anticipated.
The latest clashes began on Thursday after the northern army said 22 of its men were killed in a southern ambush.
The UN said the northern troops were being escorted out of Abyei by UN peacekeepers.
UN officials described the incident as "a criminal attack" and the US called on South Sudan to "account" for the assault.
Washington said the attack was "in direct violation" of the agreement signed by the north and south in January to "remove all unauthorised forces" from Abyei.
South Sudanese forces denied responsibility for the incident.
However, the north warned it was prepared to retaliate, and then did so with artillery fire and greater military strength than the southerners could muster.
Abyei is claimed by a southern group, the Dinka Ngok, and northern nomads, the Misseriya.
Analysts have predicted that the dispute between the two groups has the potential to drag in the northern and southern armies.
source : bbc.co.uk
