Sudan's army has launched repeated air strikes on the southern army in Unity state in a bid to seize oilfields there weeks before the south's independence, a southern army spokesman said.
Philip Aguer, spokesman for the Sudan People's Liberation Army (SPLA) of the south, said the SPLA was on "maximum alert" and strengthening its defensive positions, fearing the start of an invasion to seize the oilfields.
"SAF aircraft bombed the area of Yau, in Unity state, many times on Thursday," Aguer told AFP, referring to the north's Sudanese Armed Forces.
"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 oilfields."
But a UN spokeswoman denied that the northern army had launched air strikes south of the border.
"The place that they bombed was an SPLA assembly area, right on the north-south border. This is one of the disputed territories," Hua Jiang for the UN mission in Sudan told AFP. A Sudanese army spokesman was not immediately available for comment.
Heavy clashes between SAF troops and northern members of the former southern rebel army first erupted in South Kordofan, the adjacent state north of the border, on Sunday.
The heavily armed state retains strong links to the south, especially among the indigenous Nuba peoples who fought on the side of the southern rebels, even though their homeland, the Nuba Mountains, is in the north.
Earlier Friday, the governor of South Kordofan, Ahmed Harun, accused two key figures in the northern branch of the SPLM, the southern army's political wing, of causing this week's fighting.
Abdelaziz al-Hilu and Yasser Arman "bear responsibility for what has happened in the state," said Harun, a stalwart of President Omar al-Bashir's ruling National Congress Party, speaking on the state-owned Sudanese Radio.
While the SPLA was in control of certain areas, including Um Dorain and Kauda, 80 percent of the state was unaffected by the fighting, he added.
"I do not see the opportunity for dialogue on the horizon," said Harun. Like Bashir, he is wanted by the International Criminal Court on charges of war crimes committed in Sudan's western Darfur region.
Harun was re-elected governor last month in a bitterly disputed election that pitted him against Hilu, his former deputy and number two in the SPLM north, who pulled out of the race alleging fraud.
Arman told AFP on Tuesday that what had sparked the conflict had been Khartoum's unilateral security decisions, particularly its forcible disarmament of SPLA troops in South Kordofan.
The United Nations warned earlier that the fighting had spread right across the volatile border state, raising the prospect of direct conflict between north and south Sudan ahead of southern independence on July 9.
Bombing and heavy artillery fire was again heard early on Friday around South Kordofan's capital Kadugli, where the UN spokeswoman said the northern army was observed reinforcing its military positions.
The United States on Friday blamed the north for the renewed violence.
"The United States condemns reported acts of violence in Southern Kordofan that target individuals based on their ethnicity and political affiliation," White House spokesman Jay Carney said.
"The government of Sudan must prevent further escalation of this crisis by ceasing immediately its pursuit of a military solution to disarm" the SPLA, he added.
The UN Office for the Coordination of Humanitarian Affairs said in Geneva on Friday that up to 40,000 people were now estimated to have been displaced by the fighting in Kadugli alone.
UN officials estimate another 100,000 people, most of them ethnically southern Dinka Ngok farmers, have fled to the south from the contested Abyei border region nearby since it was overrun by northern troops on May 21.
South Kordofan is north Sudan's only oil-producing state.
It accounts for around 25 percent of Sudan's total output of around 480,000 barrels per day, meaning Khartoum will see a sharp fall in its vital oil revenues when the south secedes unless an amicable revenue-sharing agreement is reached. That now seems unlikely.