A refugee camp in South Sudan's Unity state was bombed on Thursday, South Sudan officials and witnesses said, threatening to raise tensions with Sudan.
Sudan's armed forces denied they carried out the strike. South Sudan's president Salva Kiir talked of Sudan's pending invasion of its neighbour.
Violence along the poorly defined border since South Sudan's independence in July has strained ties between the two nations. They have accused one another of backing rebel groups on their sides of the border.
A Reuters correspondent heard a large explosion in the Yida refugee camp, then saw a crater about two metres (6.6 feet) wide, an unexploded bomb wedged in the side of a school building and a white aircraft flying north. Witnesses said there were three further explosions at 3 p.m. (1200 GMT).
Yida is a camp of about 20,000 refugees from the Nuba mountains region of South Kordofan, a state north of the border where rebels have been fighting Sudan's army since June. The camp is less than 25 km (15 miles) from the border with Sudan.
There were no immediate reports of any casualties.
Taban Deng, the governor of Unity state, accused Khartoum of carrying out the attack. These people (Khartoum) should be taken to book. They should adhere to international laws and regulations, Deng told reporters in Bentiu.
The refugees need to be safe and need to be protected. They ran away from war. They should not be pursued inside the territory of South Sudan, he said.
Al-Sawarmi Khalid, spokesman for Sudan's armed forces, said Sudan had not bombed anywhere 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.
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.
Fire crackled in the dry grass around the crater about 100 metres away from an aid agency compound in Yida camp.
They (Khartoum) don't want any life in the Nuba mountains, and now they are expanding the war to the South Sudan republic, said Yousif Ismail, a refugee from the Nuba mountains.
Fighting has 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.
Last week, Khartoum submitted its second complaint to the U.N. Security Council, accusing South Sudan of supplying anti-aircraft and anti-tank missiles, ammunition, landmines and mortars to the insurgents.
Kiir denied those claims in a statement on Thursday, calling them smoke screens to mask Khartoum's own activities in support of the armed dissident groups that are fighting its proxy war against the Republic of South Sudan.
The accusations were being used to justify Sudan's pending invasion of the South, Kiir said.
The two countries have yet to agree on how much the new nation will pay to use Sudan's oil pipelines and other facilities, which South Sudan depends on to export crude. They also dispute control of the Abyei region.
Some 2 million people died in the north-south civil war, waged for all but a few years since 1955 over religion, ideology, ethnicity and oil.