The United States has strongly condemned renewed military violence between Sudan and South Sudan and called on both sides to end the air strikes and attacks on the ground.
State Department spokeswoman Victoria Nuland on Tuesday told reporters that the U.S. is "greatly alarmed" by the renewed fighting, especially along the border area. She said the area is a flashpoint that could become even more dangerous if the violence continues.
Earlier Tuesday South Sudan accused Sudan of launching a second day of airstrikes on oil-rich territory along their disputed border. The two nations first engaged in the rare direct military confrontation on Monday.
South Sudan?s President Salva Kiir said the north's air force bombed two areas in the south's Unity state. After the bombing he says South Sudanese forces were attacked by Sudanese armed forces and the militia, but were able to repel them.
South Sudan Minister of Information Barnaba Benjamin earlier quoted the president as saying the south would not be dragged into a senseless war with Sudan.
Officials from Sudan's foreign ministry said actions taken by the Sudanese army were in response to an earlier heavy weapons attack by southern forces.
The violence comes a day after both sides accused the other's soldiers of crossing the tense, poorly marked border separating the two countries. Both sides claimed they were acting in self-defense and declared victory following the fighting. Casualty figures are not known.
After the clashes on Monday, Sudanese President Omar al-Bashir announced he was suspending an April 3 summit with South Sudan's President Kiir that had been scheduled to discuss disputes about the border and oil revenues.
U.N. Secretary-General Ban Ki-moon said he is "deeply concerned" about the clashes, and urged both sides to "peacefully address their differences." Moon also urged Kiir and Bashir to continue with the proposed April talks.
South Sudan said the fighting began Monday when Khartoum carried out a ground attack and a series of intense aerial bombardments in Unity state.
Khartoum countered by blaming the south for attacking its position in the oil-rich border region of Heglig, which is claimed by both countries.
The United Nations refugee agency expressed concern for the safety of some 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.
Some information for this report was provided by AFP and Reuters.