Sudan's military has bombed a village in an oil-rich southern region, southern officials say, as tensions increase ahead of the south's independence next month.Three people were killed in the raid on Unity State, in a move to take control of the region's oil fields, the south's military spokesman said.
Some 140,000 people have fled recent fighting near the border, the UN says.
Sudan's north-south conflict left some 1.5 million dead over two decades.
The war ended with a 2005 peace deal, under which the mainly Christian and animist south held a referendum in January on whether to secede from the largely Arabic-speaking, Muslim north.
Some 99% of voters opted for independence, and President Omar al-Bashir said he would accept the verdict of the south, where most of Sudan's oil fields lie.
But last month, his forces seized the disputed town of Abyei. There have also been recent clashes in South Kordofan state, which is in the north but is home to many pro-south communities.
Aid agencies looted South Sudan's military spokesman Philip Aguer told the AFP news agency that the bombing of Unity State was a move to seize the region's oil and said the south's military was boosting its defences.
"SAF [Sudan Armed Forces] aircraft bombed the area of Yau, in Unity state, many times on Thursday," he said, adding that the attacks had continued on Friday.
"This area is deep inside South Sudan and is a move by Khartoum to control the area and create a de facto border to control our oil fields."
The BBC's Peter Martell in South Sudan says the southern reaction suggests they are taking it very seriously indeed.
If the south's fears are proved right, our correspondent says it is a worrying development as it could trigger wider north-south clashes.
But he says it is also possible that it could be an extension of the fighting in South Kordofan, with the north bombing pro-southern fighters who had crossed into South Sudan.
Between 30,000 and 40,000 people have fled recent fighting just across the north-south border in South Kordofan, the UN says.
UN Office for the Coordination of Humanitarian Affairs spokeswoman Elisabeth Byrs said that aid agency offices had been looted in the region's capital, Kadugli and that many aid workers were among those who had fled their homes, reports the AP news agency.