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Media rights groups have called for South Sudanese authorities to release six journalists detained after footage was leaked reportedly showing President Salva Kiir urinating on himself.

The staff with the state-run South Sudan Broadcasting Corporation were arrested on Tuesday by agents from the National Security Service, according to the Committee to Protect Journalists, citing media reports and other sources familiar with the case.

They are under investigation over a video that went viral on social media in December, which appeared to show Kiir urinating on himself at an official function, according to the reports, the New York-based CPJ said in a statement issued late Friday.

The arrests match "a pattern of security personnel resorting to arbitrary detention whenever officials deem coverage unfavourable", said CPJ's sub-Saharan Africa representative Muthoki Mumo.

"Authorities should unconditionally release these six SSBC employees and ensure that they can work without further intimidation or threat of arrest".

The Union of Journalists of South Sudan also called for a "speedy conclusion" of the investigation into the six, who it said were suspected of "having knowledge of the release of 'a certain footage' to the public".

"If there is a prima facie case of professional misconduct or offence then let authorities expedite an administrative or legal process to address the issue in a fair, transparent (manner) and in accordance with the law," it said in a statement on Friday.

Kiir, 71, oversaw the birth of South Sudan as an independent nation after it broke free from Sudan in July 2011.

But the world's youngest country has lurched from crisis to crisis since then, enduring brutal conflict, political turmoil, natural disasters and hunger.