Clashes first broke out on Monday in the worst direct confrontation between the two since South Sudan became independent in July 2011 but died down two days later when South Sudanese troops moved out of the disputed Heglig area, inside Sudan.

But on Friday Sudan launched an aerial bombardment on South Sudanese army border positions, according to South Sudan's army. A Sudanese army spokesman in Khartoum said it attacked with artillery, not aircraft, and only in response to an earlier South Sudanese artillery attack on Heglig.

The United Nations and the United States have both warned that the clashes could reignite a civil war that stretched for decades between the mainly Muslim north and the Christian and animist South.

The Heglig field is key to Sudan's economy because it accounts for around half of the 115,000 barrels of oil Sudan produces each day. The field was awarded to Sudan by the Permanent Arbitration Court in 2009 but some southern officials have laid claim on it.

At the African Union headquarters in Addis Ababa, the first set of talks since violence erupted were due to begin on Saturday but would now not take place until at least Sunday, diplomats there said.

"We are here and we are ready to talk," Idris Abdelgadir, head of Sudan's negotiation team, told Reuters as he arrived, but his counterpart accused Khartoum of delaying.

"We are still waiting for talks but they never showed up," Juba's top negotiator Pagan Amum told Reuters. "That's because they are planning to carry out more attacks on South Sudan."

Diplomats see no breakthrough after Sudan's President Omar Hassan al-Bashir called off an April 3 summit with his southern counterpart Salva Kiir, due to the violence.

A diplomat and Sudanese source said Khartoum was ready to talk about rescheduling the presidents' summit but nothing had been decided yet.

GUERILLA ATTACK

Sudanese army spokesman Sawarmi Khaled Saad said the SPLA, South Sudan's army, was also supporting rebels of the SPLM-North in an attack on the town of Talodi in South Kordofan by covertly slipping regular soldiers over the border.

Philip Aguer, spokesman for the SPLA denied it was supporting the rebel attack.

The Sudanese states of South Kordofan and Blue Nile are home to populations which sided with the south during the civil war but were included in Sudan when the border was drawn. The Sudanese army has been fighting SPLM-North rebels in both states since last year.

Sudan holds air superiority over South Sudan and has greater land firepower than the SPLA - an army drawn from former rebel militias created during the civil war.

(Writing by Ulf Laessing; Additional reporting by Khaled Abdelaziz; Editing by Ben Harding)