South Sudan's government urged opposition forces to halt fighting on Tuesday, saying a rebel advance and ongoing clashes that have already caused mass displacement in Jonglei state threatened to reignite civil war.

The clashes pitting government forces against fighters loyal to the Sudan People’s Liberation Army‑in‑Opposition (SPLA‑IO) are occurring at a scale not seen since 2017, according to the United Nations.

The government said its forces had repelled rebel advances in Jonglei, which stretches from the border with Ethiopia to central South Sudan.

“The ongoing security operation in Northern Jonglei State is a lawful and necessary measure aimed at halting the advance of rebel forces, restoring public order, and safeguarding civilians,” information minister and government spokesperson Ateny Wek Ateny said in a statement.

"The Government calls upon the SPLM/A in Opposition to immediately cease hostilities... Any actions that undermine the (2018) Agreement pose a serious threat to peace and jeopardise the ongoing transitional process," he added.

South Sudan's military on Sunday ordered all civilians and personnel from the UN Mission in South Sudan (UNMISS) and other charities to evacuate three counties in Jonglei ahead of its operation against opposition forces.

A spokesman for the UN Secretary‑General Antonio Guterres said the mission had shared concerns that the fighting could put hundreds of thousands of civilians at risk.

"The mission also warns that escalating hate speech is fuelling ethnic tensions and risks drawing civilian communities into the conflict," the spokesperson said.

UNMISS on Sunday said at least 180 000 people in Jonglei had already been displaced by the fighting.

The 2013-18 civil war between President Salva Kiir's forces and those loyal to his vice president, Riek Machar, was fought along largely ethnic lines and cost about 400 000 lives.

Machar is currently on trial for treason after an ethnic militia with historic ties to the SPLA-IO overran an army base in the northeastern town of Nasir last year.

He has denied the charges.