The South Sudanese embassy has not been given access to one of its citizens detained in Sudan along the tense southern border with three other foreigners, Juba's ambassador said on Thursday.
Sudan's army said on Saturday it arrested the South Sudanese, a South African, a Norwegian and a Briton in the Heglig oil region as they collected "war debris for investigation," and suggested the men were working in support of South Sudan in its "aggression" against the north.
Their employers say they were on a demining mission on the South Sudanese side of the border.
South Sudan's ambassador in Khartoum, Kau Nak, told AFP that while the other detainees have been able to meet diplomats from their countries, he does not even have a name for the captive South Sudanese.
"We wrote a note verbale and they have not responded," he said, referring to the Sudanese side. "Only me who is not given access. I don't see a reason."
South Africa's ambassador expressed hope on Thursday for a quick resolution of the foreigners' case.
"We hope that there is a speedy conclusion of this matter," Graham Maitland told AFP after meeting the captured South African, Thabo Siave.
"We've got the access, which is good, and they are in good condition. We are doing the consular processes for them," he said.
Jan Ledang, country director for the Norwegian People's Aid mission in South Sudan, has identified the Norwegian captive John Sorbo as one of his employees.
A South African demining company said two of its workers, Siave and the South Sudanese, were among the group abducted by the Sudanese military while on a UN landmine clearance contract in South Sudan.
Sudan's foreign ministry says it told the Norwegian, British and South African ambassadors that their citizens are under investigation because they illegally crossed the border into a military area, and had military equipment with them.
Nationalist feeling has intensified in Sudan after South Sudan seized and occupied the north's main Heglig oilfield for 10 days, a move that coincided with Sudanese air strikes against the South.
It was the most serious fighting since the South's independence last July, and raised fears of a wider war.
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 in line with international calls.