The United Nations and the African Union are considering reducing the size of their joint peacekeeping mission in Darfur, citing improving conditions even as violence flares along the Sudan/South Sudan border.

Officials at the African Union headquarters in Addis Ababa said Wednesday that the size of the cutback has not been decided yet, but it is prompted by improved security conditions in the Darfur region and the return of refugees to their homes.

However the African Union has expressed “deep concern” about the security situation along the border between Sudan and South Sudan, where air strikes and ground attacks have taken place this week. It called on both sides to pull back 10 kilometers from the area where the border is disputed.

In a statement late Tuesday, A.U. chairman Jean Ping called for both sides to respect their recently signed agreements and said any disputes must be resolved through peaceful means. He noted the “unfortunate and worrying” development comes just before Sudan and South Sudan's presidents are scheduled to meet to discuss border and oil revenue disputes.

Sudanese President Omar al-Bashir has said he is suspending the planned April 3 summit with South Sudan's President Salva Kiir because of the violence.

Tuesday, South Sudan accused Sudan of launching a second day of airstrikes on oil-rich territory along their disputed border. But Sudan denied the charge, and a spokesman for Sudan's ruling National Congress Party, Rabie Abdelati Obeid, said his country had merely responded to an incursion by South Sudan in its territory.

“They came close to the petroleum area, which [is] about four-kilometers inside the region, which is belonging to the north. And that is why our government chased them far away from the area.”

U.S. State Department spokeswoman Victoria Nuland said Tuesday the United States is “greatly alarmed” by the renewed fighting, especially along the border area.

U.N. Secretary-General Ban Ki-moon said he is “deeply concerned” about the clashes, and urged both sides to “peacefully address their differences.” He also urged Mr. Kiir and Mr. Bashir to continue with the proposed April talks.

The U.N. refugee agency expressed concern for the safety of 16,000 Sudanese refugees that recently fled the Nuba mountains to South Sudan's Yida settlement.

A spokeswoman says the area is not safe due to its proximity to the volatile border area.

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 is refusing 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 financially.

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