Some 45 southern Sudanese soldiers were under siege by northern Sudanese forces who have encircled them, a southern army spokesperson said on Friday.

"This is a very serious development," said spokesman Kuol Diem Kuol "Our forces have been trapped there since yesterday," he added, referring to a stand-off close to the centre of Africa's largest country.

A north-south peace agreement in 2005 ended more than 20 years of war and allowed for the south to maintain an army - the former rebel Southern People's Liberation Army (SPLA) - together with the north's Sudan Armed Forces (SAF).

Militias who were operating under the support of either side during the conflict have to choose to join either the SPLA or SAF, according to the peace deal.

The SPLA platoon was moving across Southern Kordofan State near the centre of Sudan when it was surrounded, Kuol said.

There was no immediate comment from the SAF.