By Alex Dziadosz and Khalid Abdelaziz
KHARTOUM (Reuters) - Fighting between northern Sudanese forces and armed groups in the tense Southern Kordofan border state for a fourth day Tuesday forced many residents to flee the state capital Kadugli, witnesses said.
Sudan's south is due to secede on July 9 and tensions are high after Khartoum seized the disputed Abyei region on May 21.
Two witnesses in Kadugli said they had heard heavy shooting Tuesday and said many residents were fleeing on foot.
"The sound of gunfire is coming from everywhere," one witness said, adding he had seen a large number of tanks and armed forces entering the center of the city.
A spokesman for the northern army was not immediately available to comment, but a statement published on the Sudanese Media Center website Tuesday said the army had not carried out any military operations inside Kadugli.
The Nuba mountains region of Southern Kordofan state, which is in northern territory, is home to many fighters who sided against Khartoum during the last civil war. The fighters are still referred to as members of the Sudan People's Liberation Army (SPLA), the southern military, although the government in Juba says the militias are no longer part of its army.
Six bodies have arrived in the police hospital in Southern Kordofan's capital Kadugli, including two civilians and four northern government policemen, the United Nations Mission in Sudan (UNMIS) said.
"UNMIS is concerned about the ongoing fighting between SAF (Sudanese Armed forces) and SPLA, and the deteriorating security situation in Kadugli," UNMIS spokesman Kouider Zerrouk said.
He said the United Nations had moved international staff out of Kadguli to a UNMIS compound outside the town, adding that UNMIS and the International Organization for Migration would provide shelter for civilians who fled the fighting.
Khartoum has threatened to disarm or clear out militias in Southern Kordofan, which borders Abyei and the western Darfur region, where rebel groups are fighting government forces.
The southern army says the fighters are not theirs and that it cannot ask them to withdraw south because they are northern.
"There is no relation between the government in Juba and the SPLA in the Nuba mountains or anywhere in the north," SPLA spokesman Philip Aguer said.
"If the north attacks them, it will be another situation like Darfur, with the north attacking their own people."
Southern Kordofan has the most productive oil fields that will be left in the north after the split, which could see Khartoum lose up to 75 percent of Sudan's 500,000 barrels per day of oil output.
The fighting there follows the north's occupation of the fertile, oil-producing Abyei region, which both sides claim, more than two weeks ago.
The north moved tanks and troops into Abyei after an attack on northern soldiers and U.N. peacekeepers blamed on southern forces. Tens of thousands of people fled fighting and looting.
Khartoum has since refused to withdraw from Abyei, defying calls from the United Nations, United States and southern officials. It says the land belongs to the north and its troops are there to maintain security until a solution is found.
North and south have not yet agreed on issues including oil revenue-sharing and the position of the common boundary.
(Additional reporting by Jeremy Clarke in Juba; Editing by Louise Ireland)