Sudan has declared a state of emergency in areas bordering South Sudan, giving authorities wide powers of arrest a day after they detained three foreigners in a flashpoint town along the frontier.

The detentions and Sunday's state of emergency declaration heightened tensions even further along the border between the old rivals, who in the past month came to the brink of an all-out war because of renewed fighting in disputed areas.

Sudanese officials have accused South Sudan of using foreign fighters during its assault on the oil-rich Heglig region, which Sudan claims. Southern Sudanese troops briefly captured the area, amid rising international concerns of an escalation in the fighting between the two countries.

Sudanese army spokesman Colonel Sawarmy Khaled claimed on state television late on Saturday that four people arrested in the Heglig region, including a Briton, a Norwegian, a South African and a South Sudanese, had military backgrounds. He alleged they were carrying out military activities in Heglig, but did not elaborate. Khaled said the arrests prove its government claims that South Sudan uses foreign fighters.

But a representative for one of the three said on Sunday that they were on a humanitarian mine-clearing mission.

South Sudan split from Sudan in July last year, but the two countries have yet to agree on border demarcation and divvying up oil revenues and resources.

South Sudan invaded Heglig earlier this month, saying it belonged to the south. Sudan later retook the town; Sudanese forces say they pushed out the South Sudanese while South Sudan says its troops pulled out to avoid an all-out war. Sudan elevated the tension even further by bombing South Sudan.

In Oslo, a Norwegian humanitarian organization said on Sunday that one of its employees, 50-year-old John Soerboe, was detained while on a five-day mine-clearing mission in southern Sudan with the Briton and South African.

The group denied he was on a military mission and said he had been working for more than seven years to clear the region of mines.

The Norwegian People's Aid organization called Soerboe "one of our most experienced aid workers". Per Nergaard, the group's head of emergency preparedness, said Soerboe used to be in the Norwegian military years ago before turning to humanitarian work. He had been working in southern Sudan since 2005.

He said in a statement on the group's website that Soerboe was on a "routine" mission, with the representatives from South Sudan and UN anti-mine organizations, in a region that borders Sudan.

Nergaard did not know the names of the others arrested, or have details about the incident. They were taken by Sudanese authorities to Khartoum, he said.

"The circumstance surrounding their arrest and exact location at the time is yet unclear," he said. "Our main priority now is to ensure that Soerboe and his colleagues are safe and to assure their rapid release. We are working closely with the Norwegian Foreign Ministry and our UN partners to assure this," Nergaard said.

The Norwegian organization has been working in the area since 1986, he added.