The last two South Sudanese journalists of a group of seven detained over a video purportedly showing President Salva Kiir urinating on himself have been freed, a media lobby group said on Friday.

The seven journalists -- staff at the state-run South Sudan Broadcasting Corporation -- were arrested in January by agents from the National Security Service as part of an investigation into the source of the clip, which went viral on social media in December.

In the footage, 71-year-old Kiir, dressed in his trademark black hat and a grey outfit at what is described as a road commissioning ceremony, is seen with a damp patch on his left trouser leg.

On Friday, the Union of Journalists of South Sudan (UJOSS) welcomed the release of reporters Garang John and Jacob Benjamin and said it would continue fighting "to ensure journalists work in a free and safe environment".

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

But the world's youngest country has lurched from crisis to crisis since then, enduring conflict, political turmoil, natural disasters and hunger. Between 2013 and 2018, 400,000 people died and millions were displaced.

South Sudan ranks 128th out of 180 countries on the Reporters Without Borders (RSF) press freedom index.

The media watchdog says freedom of the press is "extremely precarious" in South Sudan, "where journalists work under constant threat and intimidation, and where censorship is ever present".