A South African demining company on Sunday said two of its workers were abducted by the Sudanese military while on a UN landmine clearance contract in South Sudan.

Ashley Williams, CEO of state-owned Mechem, said its employees, a South African and a local South Sudanese, were abducted with a British UN employee and a Norwegian.

Williams rejected suggestions by the Sudanese army spokesman that the men were working in support of South Sudan in its "aggression" against the north.

"It's humanitarian work so the story of them being military advisers and this type of thing is completely and utterly nonsense and not true," said Williams.

"We are doing humanitarian landmine clearance on a UN contract and our members have full UN immunity. The abduction took place well within South Sudan territory," he told AFP, saying the group were travelling south between two UN bases.

"Then they grabbed them and drove back to Heglig with them where they then said they've arrested them in this disputed area while they weren't there at all."

A team remained in the area, which the United Nations would bring out with protection over fears of similar action, Williams said.

Sudanese army spokesman Sawarmi Khaled Saad on Saturday said the group were captured within Sudan's borders in the tense Heglig oil area.

"This confirms what we said before, that South Sudan in its aggression against Heglig was supported by foreign experts," he told reporters after the four were flown to the capital Khartoum.

"We captured them inside Sudan's borders, in the Heglig area, and they were collecting war debris for investigation," Saad said.

He added that all four had military backgrounds, and were accompanied by military equipment and a military vehicle. He did not elaborate.

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.

Jan Ledang, country director for the Norwegian People's Aid (NPA) mission in South Sudan, identified one of the captives as its 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 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.

Norway's ambassador to Sudan Jens-Petter Kjemprud told AFP on Sunday that his mission still had not obtained consular access to its citizen.

A British embassy spokesperson said her mission was "urgently investigating the arrest of a British national in Sudan" and had requested consular access.

South Sudan's charge d'affaires in Khartoum, Kau Nak, told AFP he did not have the name of the detained South Sudanese, whom he assumed to be a driver.

"That person is not an SPLA officer or even a soldier," he said, referring to the Sudan People's Liberation Army of the South.

He said the SPLA would not have allowed anybody to enter the Heglig area.

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.

The South Sudanese army said on Saturday that it had repelled an attack by Sudanese-backed rebels outside Malakal, a border town in the breakaway country's Upper Nile state.