Fighting has escalated in war-torn South Sudan forcing up to 100,000 people to flee their homes, the United Nations has said.

Toby Lanzer, the UN's humanitarian coordinator for South Sudan said up to 100,000 people had been displaced from their homes in Unity State, as clashes intensified between rebels and government troops.

South Sudan: Country of Dreams (Part 1)

"Since the beginning of May, military activities south of Bentiu in Unity State have forced up to 100,000 people from their homes," Lanzer said in a statement.

"People should never be harmed, and certainly not targeted or forced to flee from their homes," he added.

Also on Saturday, two global aid agencies evacuated their international staff from part of Unity State fearing clashes.

Doctors Without Borders (MSF) and the International Committee of the Red Cross (ICRC) said they withdrew from the town of Leer, Machar's hometown, over concerns of an "imminent attack".

"Today, we withdraw again with a heavy heart, because we know how civilians will suffer when they are cut off from critical, lifesaving medical care," Paul Critchley, head of mission at MSF said.

MSF was previously forced to abandon Leer in January last year when fighting over the town made it too dangerous to stay.

South Sudan: Country of Dreams (Part 2)

When aid workers were able to return four months later they found the hospital burned and looted and vehicles stolen.

Franz Rauchenstein, the head of the ICRC in South Sudan, urged the warring sides to respect international law.

"At all times, those who do not take part in the hostilities must be spared and the distinction needs to be made between civilian objectives and military objectives," he said.

Violence in the world's youngest nation has been characterised by rape, attacks on civilians and medical facilities and ethnic massacres.

Tens of thousands of people are believed to have been killed since South Sudan gained independence from Sudan in 2011.

Source: Agencies