South Sudan's military has ordered all civilians and personnel from the UN mission and all other charities to evacuate three counties in Jonglei State ahead of an operation there against opposition forces.

Clashes that the United Nations says are occurring at a scale not seen since 2017 have been convulsing South Sudan, Africa's youngest country, for months.

Some of the fiercest fighting has taken place in Jonglei, located in the country's east on the border with Ethiopia, where the South Sudan People's Defence Forces (SSPDF) is seeking to halt an offensive by fighters loyal to Sudan People's Liberation Army-in-Opposition (SPLA-IO).

An operation code-named "operation enduring peace" was "imminent," the SSPDF said in a statement on Sunday.

The military said all civilians living in Nyirol, Uror and Akobo counties in Jonglei were "directed to immediately evacuate for safety to government-controlled areas as soon as possible."

All personnel from the United Nations Mission in South Sudan (UNMISS) and those working for non-governmental organisations were also ordered to evacuate the three counties within 48 hours.

"Our peacekeepers in Akobo remain in place, carrying out all efforts under our mandate to help de-escalate tensions and prevent conflict," a UNMISS spokesperson told Reuters. She did not say whether UN staff also remained in the other counties.

Last week SPLA-IO called on its forces to march on South Sudan's capital Juba, signalling a major escalation.

Earlier this month SPLA-IO forces seized the town of Pajut in heavy fighting in the north of Jonglei and the town's capture was seen as putting the state capital of Bor at risk.

In a statement on Sunday UNMISS said 180,000 people in the state had already been displaced by the conflict and urged South Sudan's leaders "to put the interests of their people first by stopping the fighting."

Medical charity Médecins Sans Frontières (MSF) said in a statement on Sunday it had evacuated key staff from Akobo county after "clear instruction from the relevant authorities, and in response to the deteriorating security situation in the area."

SPLA-IO forces led by South Sudan's vice president Riek Machar battled the military in the 2013-18 civil war, which was fought along largely ethnic lines and killed about 400,000 people.

A peace deal in 2018 quieted the conflict, although localised clashes have persisted.