The family and rights groups have expressed concern for former Juba mayor Kalisto Ladu, whose whereabouts have not been known since security agents arrested him on Saturday night.

Mr Kalisto, a former mayor of Juba City Council, is a vocal critic of land policy in South Sudan.

Security agents have reportedly been irked by Kalisto's recent speech during a funeral rites service in Juba, where he decried land-grabbing practices in Juba County.

Juan Andrea Fiorentino, the wife of Kalisto Ladu, told Radio Tamazuj Monday evening that the authorities have not yet informed them about Kalisto's whereabouts. "Since Kalisto was arrested on Saturday night until now, nobody came or called to inform us about his whereabouts," she said.

Juan has appealed to the South Sudan authorities to reveal her husband's whereabouts or release him immediately.

"We are very concerned as a family. So my message is that let them release him because he has little children and is the family's breadwinner. The situation of the family, especially the kids, is bad because I don't have a job," she said.

Juan appealed to the authorities to either release her husband or take him to a court of law if he had committed a crime. "The information we received from sources last week is that some people were looking for Kalisto because of his recent speech that was circulating on social media," she said.

South Sudan's constitution requires detainees to be produced before a court within 24 hours. Despite this, authorities routinely detain suspects for long periods, including during investigations and subsequent trials.

The National Security Service law gives agents broad powers to arrest and detain citizens without warrant or charge.