The British embassy was "urgently" investigating on Sunday the arrest in Sudan of one of its citizens, who was among four foreigners the Sudanese military said it captured in the tense Heglig oil region.
"We are urgently investigating the arrest of a British national in Sudan," an embassy spokesperson told AFP. "We immediately requested consular access".
Sudanese army spokesman, Sawarmi Khaled Saad, on Saturday identified the foreigners as a Briton, a Norwegian, a South African and a South Sudanese.
"We captured them inside Sudan's borders, in the Heglig area, and they were collecting war debris for investigation," Saad said after the four were brought to the capital Khartoum.
A colleague of one of the men said they were deminers working on the South Sudanese side of the border.
Jan Ledang, country director for the Norwegian People's Aid (NPA) mission in South Sudan, identified one of the captives as employee John Sorbo.
"It's impossible that they were in Heglig -- they were in Pariang" about a 90-minute drive from Heglig in the South's Unity state, Ledang said.
They were doing some follow-up demining work in the area, he added.
The four were on a de-mining mission "and one of them was from the UN", said Josephine Guerrero, a spokeswoman for the United Nations Mission in South Sudan.
"We're uncertain of the circumstances," she added.
In the most serious fighting since the South's independence, Juba's troops occupied Sudan's main oil region of Heglig for 10 days, a move which coincided with Sudanese air strikes against the South.
Sudan declared on April 20 that its troops had forced the Southern soldiers out of Heglig, but the South said it withdrew of its own accord.
Each side blames the other for damaging Heglig's oil facility, which provided about half of Sudan's oil output and is now shut pending repairs.
South Sudan broke away from Sudan in July last year after a peace deal ended one of Africa's longest civil wars, which killed about two million people between 1983 and 2005.
Tensions have risen over a series of unresolved issues including the border, the future of disputed territories and oil.
One month of clashes along the disputed frontier has raised fears of a wider war.