Four foreigners investigating the debris from recent fighting between Sudan and South Sudan have been captured in the Heglig oilfield area, Khartoum's military has said.
Sudan on Saturday said it had arrested a Norwegian, a Briton, a South African and a South Sudanese for illegally entering a disputed border area earlier on Saturday.
The men were accused of helping the South Sudan army, a claim South Sudan has denied.
"We managed today to arrest foreigners; a Briton, a Norwegian, a South Sudanese and a South African," Al-Sawarmi Khalid, a spokesperson for the Sudanese army, said.
"They were arrested inside Heglig area inside the Sudanese borders while they were doing suspicious acts represented by collecting the remains of the war and other war-related works besides holding some military materials.
"They were brought for investigation and all the information will be released later after their interrogation."
Josephine Guerrero, a spokeswoman for the UN mission, said: "Four people, including one UN staff member, were taken to Khartoum." She declined to give any further details.
Al Jazeera's Zeina Khodr, reporting from Khartoum, said that reports are stating the men were staff members of NGOs rather than spies.
"I spoke to a government official last night, who told me that regardless of who these men were or who they worked for, what were they doing on Sudan's territory?" she said, adding that it only goes to confirm the level of distrust between the two nations.
Regarding the level of tension between the two sides, our correspondent said that "in the absence of talks, both sides are using oil, and the economy, to exert pressure."
"At the same time, both sides cannot afford a war."
Jay Ledang, country director for Norwegian People's Aid, told Al Jazeera from Juba that they were seriously concerned that one of their senior mine-action staff had been taken into custody in Khartoum, adding that the circumstances around the incident still cannot be confirmed.
"We have no activities in Heglig so [the claims from Khartoum] are totally nonsense," he said.
"[The aid workers] were on the South Sudan side, and doing some quality assurances. What really happened we cannot say," but they did not cross the border, intentionally if this is the case, that is for sure, said Ledang.
A Reuters witness saw four men arriving on a civilian plane at Khartoum's military airport. One of the men was wearing a t-shirt that said: "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.
Jan Ledang, the Norwegian People's Aid's South Sudan director, confirmed that one of its staff members had been detained.
|In-depth coverage of North-South strife over border
Al Jazeera's Peter Greste, reporting from Juba, said that he recognised, from the video recording of the arrested, that the arrested man from Norwegian People's Aid was a "deminer".
"We have since spoken to the Norwegian People's Aid who have confirmed that one of their deminers had been arrested along with a number of other demining workers who had been operating in that area," he said.
Greste said that they did not know any more about the circumstances of the arrest but that the organisations involved "insist that they were not on any kind of military mission".
"That they were people with military backgrounds but that they were on a demining project," said Greste.
He also said the deputy director of South Sudan's military intelligence confirmed, "that these men tried to stop the convoy of two military vehicles passing through the frontline areas", but that the "vehicles continued to go through and the men were arrested".
Greste said the deputy director also denied that the men had anything to do with the South Sudanese operation there.
South Sudan's army spokesman Philip Aguer dismissed Sudan's accusation that the men were working with his forces.
"That is rubbish and just a lie," he said, adding that southern military forces told him a UN truck had got lost in the disputed border area and were "caught by the Sudanese Armed Forces".
A British embassy official said the embassy was looking into the matter, while the South African embassy could not be immediately reached.
The UN Security Council on Thursday started talks on a resolution that could allow sanctions against Sudan and South Sudan if they do not meet the AU demands to end their fighting.
A US-drafted resolution backs the AU demands and calls for the two sides to "immediately" halt hostilities and pull their forces back into their own territory.
The text says the Security Council would review the rivals' implementation of AU demands and could "take appropriate additional measures" under article 41 of Chapter VII of the UN charter that allows for sanctions but not military force.