South Sudan is accusing Sudan of waging war and thwarting African Union peace talks aimed at stopping rising border clashes.

During talks in Addis Ababa Sunday, South Sudan's chief negotiator Pagan Amum accused Khartoum of conducting air strikes for a seventh straight day.

Meanwhile, Sudan accuses South Sudanese army forces of crossing several kilometers into the Sudan side of the Heglig oil producing region on the two countries' disputed border.

Amum denies the presence of any southern soldiers in Sudan. He also notes that Sudan's defense minister, Abdel Raheem Muhammed Hussein, missed a scheduled security meeting. He says this proves Sudan has rejected the AU-mediated talks.

“Unfortunately, the response of Khartoum is war. They have launched aerial and ground attacks from Monday. The government of Sudan is the one that is waging war, the head of the JPSM [Joint Political and Security Mechanism] on their side is not here. The meeting was supposed to take place yesterday and he has not appeared. ”

AU officials say the talks are “on hold” pending Hussein's arrival.

The presidents of the two Sudans are set to meet in Juba Tuesday to discuss border and oil revenue disputes. It is unclear if the meeting will take place, although members of the mediation panel expressed hope it will go on as scheduled.

Sudanese President Omar al-Bashir previously announced he would not attend the summit with South Sudan's President Salva Kiir because of the violence.

Negotiators say the presidents' meeting is crucial if the two sides are to resolve key issues, including sharing of oil revenue.

Both sides blame the other for sparking the direct military confrontation.

The United States, African Union and United Nations have all expressed serious concern about the fighting, and called for the parties to address their disputes through peaceful negotiations.

Since South Sudan's independence in July, the two neighbors have not been able to agree on the demarcation of their 1,800 kilometer border or how much South Sudan should pay to export oil through Sudan.

The south took over most Sudanese oil production, but refuses to pay what it considers excessive transit fees to use northern pipelines. The landlocked south needs the pipelines to send the oil to international markets.

The dispute prompted South Sudan to shut down all oil production, a move analysts say is likely to hurt both countries.

The sides are also in disagreement about the status of southerners living in the north and regularly accuse each other of supporting the other's rebel groups.