The United Nations has said that an air raid allegedly carried out by Sudan on a refugee camp on South Sudan's territory could be an "international crime".
South Sudan has accused Sudan of carrying out the attack on the camp in Yida town, less than 25km from the border with Sudan, on Thursday.
"We condemn the bombing. The attack ... could amount to an international crime," Rupert Colville, the spokesman for the UN human rights chief, said on Friday.
Miabek Lang, commissioner of Pariang County in Unity state where Yida is located, said 12 people were killed in the raid, but the claim has not been confirmed by other sources.
Earlier Taban Deng, the governor of Unity state, said: "These people [Khartoum] should be taken to book. They should adhere to international laws and regulations,"
But Al-Sawarmi Khalid, a spokesman for Sudan's armed forces, said they had not bombed any site in South Sudan's territory.
"South Sudan is a state in the United Nations. We respect international law, and it's impossible that we would do that," he said.
About 20,000 refugees are camped in the area after fleeing violence in Sudan's South Kordofan and Blue Nile states, where opposition groups have been fighting Sudan's army since June.
A Reuters correspondent heard a large explosion in Yida on Thursday, then saw a crater about two metres wide, an unexploded bomb wedged in the side of a school building and a white aircraft flying north.
Fire could be seen in the dry grass around the crater about 100 metres away from an aid agency compound in the refugee camp.
"They [Khartoum] don't want any life in the Nuba mountains, and now they are expanding the war to the South Sudan
republic," Yousif Ismail, a refugee from the Nuba mountains, said as he stood by the hole.
|The camp shelters refugees fleeing violence
in two states across the northern border
The attack threatens to raise tensions between the two former civil war foes, South Sudan and Sudan.
The US strongly condemned the attack and urged both sides to resume negotiations to prevent the violence escalating into a full-scale conflict.
"These provocative aerial bombardments greatly increase the potential for direct confrontation between Sudan and South Sudan. The United States demands the government of Sudan halt aerial bombardments immediately," Jay Carney, the White House press secretary, said in a statement.
"This bombing of civilians and humanitarian workers is an outrageous act, and those responsible must be held accountable for their actions."
Meanwhile, a US satellite monitoring group said Sudan's military is upgrading air bases near the border with South Sudan.
The Satellite Sentinel Project said on Friday that satellite imagery appeared to show the enhancement of two air bases Sudan seized in Kurmuk from rebels in Sudan's Blue Nile State.
The group said the images showed three helicopter gunships and an Antonov, the plane witnesses said was used in Thursday's bombing of the refugee camp.
Violence along the poorly defined border since South Sudan's independence in July has strained ties between the Juba and Khartoum governments. They have accused one another of backing armed groups on their sides of the border.
The Khartoum government reported fresh clashes in South Kordofan on Thursday, saying Sudanese forces fought opposition groups in near the town of Taludi, killing dozens and destroying a tank and other military vehicles.
Fighting has also broken out in Sudan's Blue Nile state this year. Blue Nile and South Kordofan are home to tens of thousands of fighters who sided with the south during the war but were left in Sudan when South Sudan seceded, analysts say.
South Sudan split off into a separate country in July after voting overwhelmingly for secession in a January referendum, the culmination of a 2005 peace deal that ended decades of war between north and south.
About two million people died in the north-south civil war, waged for all but a few years since 1955 over religion, ideology, ethnicity and oil.