Juba – South Sudan's army on Friday accused the north of bombing a southern border village, and said southern forces were getting ready to defend against a possible ground attack.
The south is preparing to secede on 9 July and fears of fresh fighting between the two long-standing rivals grew after the north seized the contested Abyei region on 21 May.
The United Nations said fighting between northern forces and southern-aligned armed groups in the north-run oil state Southern Kordofan has spread to the tip of the southern Unity state and that tens of thousands may have fled the clashes.
Philip Aguer, spokesman for the south's Sudan Peoples' Liberation Army (SPLA), said the northern military was trying to occupy areas near the border – whose exact position is yet to be decided – in an attempt to control the country's oil fields.
"There has been an air bombardment by northern forces in Unity state, in the morning yesterday and again in the afternoon. Three people were killed in the morning," Aguer said. The three dead were civilians, he said.
"We are expecting not only air attacks but also ground forces. We know their forces are moving from Abyei toward Unity state…We are getting ready to defend ourselves. Our forces near the border are on maximum alert and are expecting an attack any time," he added.
A spokesman for the northern army was not immediately available to comment. In previous statements, it has blamed southern or southern-aligned forces for provoking fighting in Abyei, Southern Kordofan and elsewhere.
South Sudan voted to secede in a January referendum promised by a 2005 peace deal that ended decades of brutal civil war between the north and south.
That vote went more smoothly and peacefully than many analysts and humanitarian groups had predicted, but a continued lack of agreement between the two sides on questions such as how to share debt and oil revenues has complicated the split.
The secession could see the north lose some 75 percent of Sudan's current 500,000 barrels a day of oil output, the lifeblood of both northern and southern economies.
Northern forces have been fighting armed groups in the volatile Southern Kordofan border state since a police station was attacked Saturday.
Between 30,000 and 40,000 people may have fled fighting in the state capital of Kadugli alone, the United Nations said on Friday. The town's normal population is estimated at 60,000.
"The fighting has spread to the disputed border area of the northernmost tip of Unity state in southern Sudan," Elisabeth Byrs, spokeswoman of the UN Office for the Coordination of Humanitarian Affairs (OCHA), told reporters in Geneva.
Armed checkpoints were set up on the main roads inside and around Kadugli and there had been reports of "large-scale" looting as late as Thursday, she added.
Analysts have seen Southern Kordofan as a flashpoint because it is home to thousands of northerners who sided with the south against Khartoum during the last civil war. Northern officials have called last week's clashes an "armed rebellion."
The region also holds the most productive oil fields that will be left in the north after the split.
Former South African President Thabo Mbeki met with Sudan's President Omar Hassan al-Bashir Thursday to discuss Southern Kordofan and Abyei and was scheduled to travel to Juba to meet officials there Friday, state news agency SUNA said.
Abyei, long a centerpiece of the north-south divide, has been one of the most contentious issues ahead of southern secession. It is used all year by the south-linked Dinka Ngok people and part of the year by northern Misseriya nomads.
Khartoum took control of area with tanks and troops nearly three weeks ago, following an attack on a convoy of northern soldiers and UN peacekeepers that was blamed on southern forces. The move drew an international outcry.
Abyei was "calm but unpredictable" with sporadic shooting in the region's main town, OCHA said Friday. Some 101,800 people may have fled the fighting there, it added, up from a previous estimate of 96,000.