South Sudan's President Salva Kiir has re-appointed his former vice president James Wani Igga to the same position, the state broadcaster said, after dismissing a powerful ally who had been widely seen as a potential successor.

Fears of a return to civil war are growing as uncertainty surrounds the succession of the 74-year-old Kiir, who has repeatedly sacked, reinstated and, sometimes, once again sacked senior officials.

Analysts see the moves as being aimed at placating diverse constituencies and navigating turbulent political moments.

Igga, a former speaker of the House, previously served as Kiir's vice president from 2013 until he was sacked in February.

Then, last week, Benjamin Bol Mel, the close Kiir ally who had held the office for about nine months was suddenly fired, along with the central bank governor and other officials.

Kiir also fired and re-appointed several other officials in Monday's decree read on state broadcaster, South Sudan Broadcasting Corp.

They included the long-serving information minister, Michael Makuei, transferred to the justice ministry while Mabior Garang De Mabior, the son of South Sudan's founding father, John Garang, was named minister for environment forestry.

South Sudan has five vice presidents under the terms of a peace pact that ended its civil war in 2018, which meant that from 2018 Igga was joined by four others including Kiir's key rival Riek Machar in the role of first vice president.

Machar is being tried on charges of treason after he was put under house arrest in March.