The South Sudanese government has released 30 political detainees in a move seen to be seeking to build consensus for national peace talks.

Jalban Obaj, the director of legal affairs at the Internal Security Bureau, said the prisoners have been set free in compliance with a presidential amnesty decreed earlier this year.

In a brief statement broadcasted by state-owned radio on Friday morning, the spy agency's official said the detainees' release was unconditional.

The government has detained hundreds of people opposed to President Salva Kiir. Rebels forces have been fighting to oust him since December 2013 when he fell out with his then deputy Riek Machar.

Many of those detained are often not charged, with human rights groups saying they are kept in "appalling" conditions.

Luka Deng Diing, an activist, welcomed the release of the political prisoners, saying it will add to the credibility of the national dialogue called by President Kiir to renegotiate August 2015 agreement.

“The release of the 30 political detainees is a positive step towards a credible national dialogue and for real peace and reconciliation in South Sudan,” he said.