Sudan's army on Wednesday confirmed it attacked an area near the South Sudanese border where Darfur rebels had set up a compound, but South Sudan said bombs landed on its territory, killing five people.
"We attacked Al-Regaibat which is 40 kilometres (25 miles) north of the international border with South Sudan and 10 kilometres north of Samaha," army spokesman Sawarmi Khaled Saad said in a written statement.
The incident adds to growing concern over delays in implementing security and oil deals which the leaders of Sudan and South Sudan hailed in September as ending conflict, after they fought along their undemarcated border in March and April.
Saad alleged that the rebels must have had "great support" from South Sudan, an accusation which goes to the heart of tensions between the two nations.
"There are five casualties, civilians: Two women, one man and two children," South Sudan's army spokesman Philip Aguer told AFP.
He said the casualties came on Tuesday when bombs fell in Kiir Adem, an area near Samaha and claimed by South Sudan.
"It's not a disputed area. It's near to the border", he said, accusing Sudan of more air raids even further south on Wednesday.
But Sudan's army spokesman insisted that "we conducted our battle deep inside Sudan."
Earlier Wednesday the Justice and Equality Movement (JEM) rebels accused the government of bombing around the disputed border region, after the army had threatened to use force.
"Yesterday and the day before yesterday there was intensive aerial bombardment around the Samaha area," said Gibril Adam Bilal, JEM's spokesman.
He said the bombs landed in civilian areas and did not hit rebel positions.
On Monday, army spokesman Saad said the Sudanese Revolutionary Front -- to which JEM belongs -- built a compound, set up a checkpoint and was flying its flag in the Al-Regaibat area, at the Bahr al-Arab River.
Sudan and South Sudan dispute the area, which Sudan considers to be part of East Darfur state.
The Sudanese Revolutionary Front is an alliance of rebel groups from different Sudanese states. It says it is fighting to overthrow the Arab-dominated Khartoum regime.
The Samaha region is one of five areas disputed by Khartoum and the South's government in Juba.
They have not been able to resolve the disagreement despite African Union (AU) mediation which led to the September deals that included a demilitarised border buffer zone designed to cut support for insurgencies in Sudan.
Sudan has long accused South Sudan of working with the JEM, a charge denied by the South. But suspected JEM fighters were seen alongside South Sudanese troops during border fighting between Sudan and South Sudan in April.
The two nations have not been able to agree on practical steps to implement the September pacts, the AU said on November 10, urging "full and timely implementation".
On Monday the US State Department spokeswoman, Victoria Nuland, called on Sudan and South Sudan to meet again and recommit themselves to the September 27 accords.
"The creation of the safe demilitarised border zone between the two countries is vital to ensure that both countries honour their commitments to cease support to proxies and, most importantly, prevent inter-state conflict," she said in a statement.
The September agreements were also supposed to allow a resumption of South Sudanese oil through northern pipelines for export.
But on Tuesday South Sudanese President Salva Kiir said restarting the flow, crucial to the fledgling nation's economy, is stalled after Sudan demanded that Juba denounce old comrades still fighting inside Sudan.
Khartoum accuses South Sudan of backing those rebels in South Kordofan and Blue Nile states.
Sudan said it is ready to receive oil from the South, which separated in July 2011 under a peace deal that ended 23 years of civil war.