Clashes between rival forces in South Sudan have displaced at least 50,000 people since February, with Western governments on Tuesday offering to mediate over a crisis fuelling fears the country will relapse into civil war.

Tensions have been mounting over clashes in the northeastern Upper Nile State between forces allied to President Salva Kiir and First Vice President Riek Machar, threatening to undermine their fragile peace-sharing agreement.

More than 20 of Machar's allies, including senior government and military officials, have been arrested in recent weeks.

"The violence is putting already vulnerable communities at greater risk and forcing the suspension of life-saving services," Anita Kiki Gbeho, an official with the United Nations Office for the Coordination of Humanitarian Affairs (OCHA) in South Sudan, said in a statement.

Deadly fighting in Upper Nile

A group of Western embassies, including the United States and European Union, called for dialogue between Kiir and Machar and said they were "prepared to facilitate their discussion".

On Monday, an air strike by the South Sudanese government in Nasir County – an area considered a stronghold of Machar supporters – killed at least 20 people, including children, area commissioner James Gatluak told AFP.

OCHA said 10,000 of the displaced had crossed into Ethiopia.

It added that 23 humanitarian workers had also been forced to leave the region and a cholera treatment unit in Nasir closed.

'On brink of relapse into civil war'

The head of the UN Mission in South Sudan (UNMISS), Nicholas Haysom, said on Tuesday the country was "poised on the brink of relapse into civil war" which threatened to undo years of peace efforts.

Neighbouring Sudan is a grave example of how quickly a nation can descend into "catastrophic war", he said, urging all parties to de-escalate tensions "before it is too late."

"This region cannot afford another conflict," he said.

Last week, Doctors Without Borders (MSF) reported 1,300 cholera cases in South Sudan's Akobo County in the Upper Nile region.

'Very worried'

Relief International staff based in the region warned they were already struggling due to massive cuts in USAID funding by the administration of President Donald Trump.

"With the supplies that we had here... it is not enough," said one member, who requested anonymity given the security situation.

The team – two doctors, six clinical officers, eight nurses and seven midwives – currently treat more than 200 patients daily at Renk Transit Centre in Upper Nile.

"So now we are very worried," said one doctor, who also asked to remain anonymous, describing staffing and medicine shortages.

Threatens 2018 peace deal

"There is no hope for refugees and returnees," he told AFP.

"When I'm seeing patients coming and there is nothing in my hand to help that patient, it is too painful."

The fighting threatens a 2018 peace deal between Kiir and Machar, who fought a five-year civil war that killed some 400,000 people.

Kiir's allies have accused Machar's forces of fomenting unrest in Nasir County in league with the White Army, a loose band of armed youths from the vice-president's Nuer ethnic community.

'Unable to return to safe locations'

Tensions spiked earlier this month when an estimated 6,000 White Army combatants overran a military encampment in Nasir.

An attempted rescue by the United Nations led to the deaths of a UN crew member and senior South Sudanese general, among others.

The statement from the Western embassies said they were concerned that survivors of the attack were "still unable to return to safe locations."