A Briton is among three foreigners arrested with a South Sudanese national in the oil-rich region of Heglig in a disputed area on the border of recently created South Sudan.

Authorities claim the group – a Briton, a Norwegian and a South African – had military hardware and an armoured vehicle, but representatives of two of those detained said they were on a mine-clearing mission.

A Sudanese army spokesman, Col Sawarmi Khalid Saad, said on state television on Saturday that the four had military backgrounds, and were carrying out military activities in Heglig, though no details were given.

But in Oslo, the Norwegian People's Aid humanitarian organisation said one of its employees, John Soerboe, was among the four. Denying he was on a "military mission", it said he had been working for more than seven years to clear the area of mines. The organisation said Soerboe was "one of our most experienced aid workers" and was in the Norwegian military before turning to humanitarian work. He had been working in southern Sudan since 2005.

A statement on the organisation's website said Soerboe was on a "routine" five-day mission in a region that borders Sudan to the north, and was travelling with representatives from South Sudan's and the UN's anti-mine organisations.

The Foreign Office confirmed a British national had been detained, and said it was "urgently investigating the details of the arrest".

"We immediately requested consular access and stand ready to provide further consular assistance," said a Foreign Office statement.

Mechem, a South African de-mining company, told Reuters news agency that two of its employees, a South African and a South Sudanese, were among those arrested.

The arrests are a sign of the increased tension along the disputed border, which has seen a spike in clashes between Sudan and South Sudan in recent weeks. The violence has brought the two nations, already at odds over demarcating the border, and dividing oil revenue, to the brink of war. Heglig was captured by South Sudanese troops earlier this month, though Sudan later said it took the region back.

Sudanese TV said the men are suspected of aiding South Sudan. South Sudan army spokesman, Philip Aguer, was quoted by Reuters as saying: "That is rubbish and just a lie."

Mechem's chief executive, Ashley Williams, said in a statement to the news agency: "We are working on a UN de-mining contract and our employers have full UN immunity."

At a press conference at Khartoum airport, the Sudanese army spokesman said: "We captured them inside Sudan's borders, in the Heglig area, and they were collecting war debris for investigation." The four men were paraded before the cameras, but not allowed to speak. One man was wearing the T-shirt of a Norwegian NGO, while another wore one with the logo of a South African de-mining group.

South Sudan became independent from Sudan after a civil war which lasted two decades and killed an estimated 1.5 million people.