South Sudan's President Salva Kiir has fired the country's army chief after seven months in the post and named a replacement, according to an announcement on state radio.

No reason was given for the firing of Paul Nang Majok in the announcement late on Monday. Majok had been in the post since December. The announcement said Kiir had appointed Dau Aturjong as the Chief of Defence Forces.

Majok was in charge of the army while fighting raged between the army and the White Army, an ethnic militia largely comprising Nuer youths, triggering the country's latest political crisis.

"There has been a tradition that when you are appointed, or reassigned there are no reasons (given) for getting appointed and there are no reasons given for getting relieved. It is normal," said Lul Ruai Koang, South Sudan army spokesperson.

South Sudan has been formally at peace since a 2018 deal ended the five-year conflict responsible for hundreds of thousands of deaths, but violence between rival communities flares frequently.

In March, First Vice President Riek Machar was put under house arrest, stirring fears of renewed conflict.

Information Minister Michael Makuei said the arrest was due to Machar contacting his supporters and "agitating them to rebel against the government with the aim of disrupting peace so that elections are not held and South Sudan goes back to war."

Machar's party has previously denied government accusations that it backs the White Army, which clashed with the army in the northeastern town of Nasir in March.

In May, South Sudan's army said it had recaptured the town from the White Army.