The Sudanese army has launched repeated air strikes on southern army positions in Unity state, less than a month ahead of the south's independence, in a bid to seize the state's oilfields, a southern army spokesman charged on Friday.
"SAF aircraft bombed the area of Yau, in Unity state, many times on Thursday," Philip 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," added the spokesman for the Sudan People's Liberation Army of the south.
Aguer said the SPLA was on "maximum alert" and strengthening its defensive positions, fearing the start of an invasion to seize the oilfields.
A UN spokeswoman, however, 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 on Friday, the governor of South Kordofan, Ahmed Harun, a stalwart of President Omar al-Bashir's ruling National Congress Party, accused two key figures in the northern branch of the SPLM, the southern army's political wing, of causing this week's fighting.
Speaking on the state-owned Sudanese Radio, he said Abdelaziz al-Hilu and Yasser Arman "bear responsibility for what has happened in the state."
He added that, while the SPLA was in control of certain areas, including Um Dorain and Kauda, 80 percent of the state was unaffected by the fighting.
"I do not see the opportunity for dialogue on the horizon," said Harun, who like Bashir is wanted by the International Criminal Court on charges of war crimes committed in the 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 Khartoum's unilateral security decisions, and particularly its forcible disarmament of SPLA troops in South Kordofan, were what had sparked the conflict.
The United Nations earlier warned 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 9 July.
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 UN Office for the Coordination of Humanitarian Affairs said in Geneva on Friday that up to 40,000 people are now estimated to have been displaced by the fighting in Kadugli alone.
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 21 May, according to UN estimates.
South Kordofan is north Sudan's only oil-producing state. It accounts for around 25 per cent 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.