North Sudan's government has rejected a call from the U.N. Security Council to pull its troops out of the disputed Abyei region.
On Friday, the Council condemned the north's occupation of Abyei, describing it as a "serious violation" of the 2005 peace deal that ended Sudan's north-south civil war.
The north's foreign minister, Ali Ahmad Karti, responded late Saturday, saying the north cannot be asked to withdraw from Abyei because it is Sudanese territory.
Both north and south Sudan claim ownership of Abyei, an oil-rich land on the north-south border.
In a separate development, the United Nations says fighting broke out Sunday in the state of South Kordofan, also on the north-south border. A U.N. spokesman in Khartoum, Hua Jiang, reports an attack on a police station in the town of Kadugli and shooting in the village of Umm Dorain.
She says U.N. peacekeepers have been sent to investigate.
The events of the past two weeks have raised fears of a new civil war in Sudan.
Last week, the north Sudan army demanded that southern troops in South Kordofan and Blue Nile states leave immediately. A southern army spokesman, Philip Aguer, denied that southern soldiers were in either state.
The northern army says Sunday's Umm Dorain incident stemmed from an individual soldier firing at random, and said the situation is now under control. It had no comment on the alleged attack in Kadugli.
The north's May 21 seizure of Abyei occurred just a few weeks before south Sudan is to formally declare independence from Khartoum.
The south voted to split from the north in a January referendum. Abyei was scheduled to hold a separate referendum on which region to join, but the poll never happened because the sides could not agree on who was eligible to vote.
Tens of thousands of Abyei residents fled the area after the northern takeover, and witnesses have reported widespread destruction of huts and buildings in Abyei's main town. The Satellite Sentinel Project, which monitors Sudan through satellite images, has accused the north of conducting "ethnic cleansing" in the area.
Some information for this report was provided by AFP.