Sudan said it had arrested a Briton, a Norwegian and a South African on Saturday, accusing them of illegally entering a disputed oil-producing border area to spy for its enemy South Sudan.
South Sudanese officials denied the allegations and said the men were working with the United Nations and aid groups clearing mines and had got lost in the remote territory close to the boundary between the two countries.
Sudanese army spokesman al-Sawarmi Khaled said the three were arrested in Heglig - the scene of recent fighting between Sudan and South Sudan - travelling with a South Sudanese soldier in vehicles carrying military equipment.
"It is now confirmed without any doubt that South Sudan used the help of foreigners in their attack on Heglig. These foreigners were doing military work such as spying out the areas ... They had military equipment ... They have a military background," Sawarmi said.
The group had been flown to Khartoum, he added.
A Reuters witness saw four men arriving on a civilian plane at Khartoum's military airport.
One of the men, a Westerner, was wearing a t-shirt marked with the slogan "Norwegian People's Aid. Mine Action South Africa". Reporters were not allowed to talk to the men who were swiftly driven away in an unmarked white van.
South Sudanese Information Minister Barnaba Benjamin dismissed the Sudanese account as "nonsense", telling Reuters the men were workers for aid groups and the United Nations and had been clearing mines.
South Sudan's army spokesman Philip Aguer said military sources had told him a U.N. truck had got lost after leaving Paryang, just north of Bentiu, the capital of South Sudan's Unity state, and was "caught by the Sudanese Armed Forces".
Britain's Foreign Office in London confirmed the Briton's arrest and Norwegian People's Aid (NPA) South Sudan director Jan Ledang said one of its staff members had been detained.
"We are trying to confirm the nationalities of the three and the aim and motivation of the three," Norway's ambassador to Sudan, Jens-Petter Kjemprud, told Reuters.
MECHEM, a demining company and an arm of South Africa's state arms company, Denel, said two of its employees, a South African and a South Sudanese, were arrested along with a United Nations employee.
"We are working on a UN demining contract and our employees have full UN immunity," MECHEM's chief executive officer, Ashley Williams, said in a statement emailed to Reuters.
The United Nations mission in South Sudan said one of its officials had been taken to Khartoum with three other men, without going into further detail.
New Bombing Accusations
More than three weeks of border fighting between Sudan and South Sudan's 1,800 km (1,100 mile) contested border has brought the African neighbours close to an all-out war, nine months after the South gained independence from Sudan under a 2005 settlement.
South Sudan's army seized Heglig earlier this month but announced a withdrawal more than a week ago, bowing to pressure from the United Nations.
Benjamin said Sudan's war jets dropped eight bombs on Panakuach in Unity state on Saturday. Sudan's army could not be immediately reached for comment.
Benjamin also said two SPLA soldiers has been killed on Friday after South Sudan repulsed an attack by what it said was a Sudanese-backed rebel militia near Malakal in its Upper Nile state.
China and the African Union (AU) have stepped up diplomatic efforts to try and bring Juba and Khartoum back to the negotiating table.
The United States circulated on Thursday a draft resolution at the UN Security Council that warns both states of sanctions if they do not comply with an AU
seven-point peace plan. The deal urges both sides to cease hostilities within 48 hours and to withdraw troops from disputed areas.
The dispute has already halted most oil production in the two countries, damaging their fragile economies.