The ministry, which was informed of Križnar's arrest about a week ago by his wife, revealed no other detail, probably for Križnar's safety.

There have been no official reports on what grounds he was arrested, but Jožica Bračun Bokor, who is in charge of protection of Slovenian citizens abroad at the ministry, said on Friday that Križnar's cameras had allegedly been seized and that he had no permit for travel in the area where he was arrested.

Immediately after learning about the arrest, the Foreign Ministry contacted the Slovenian Embassy in Cairo, which asked the German Embassy in South Sudan for consular protection.

Subsequently, Križnar was visited at a Juba prison by a German diplomat who served as his contact with Slovenia.

The Slovenian ministry sent two diplomatic to the South Sudan authorities, asking for the case to be resolved as soon as possible.

This was not the first time that the 61-year-old activist and humanitarian worker was incarcerated in Sudan.

In 2006 he spent about a month in a prison in El Fashir after being arrested for trying to enter the country without a visa.

He was released after a petition was lodged by the then President Janez Drnovšek, for whom Križnar served as an envoy for Sudan.

In 2003, he was incarcerated in Ethiopia for being in a restricted movement zone without the necessary permit.