The United Nations on Tuesday said around 75 bodies had been discovered in a mass grave in rebel-held Bentiu, the capital of South Sudan's oil-rich Unity State, following a surge of violence in the world's youngest nation.

"We have discovered a mass grave in Bentiu, in Unity State, and there are reportedly at least two other mass graves in Juba," UN rights chief Navi Pillay said in a statement.

Pillay's spokeswoman told AFP the mass grave in Bentiu contained around 75 bodies.

Pillay expressed "grave concern" over the ethnically-tinged killings that have raged across South Sudan for more than a week as troops loyal to President Salva Kiir battle those backing his rival Riek Machar, a former vice president who was sacked in July.

The official toll has stood at 500 dead for days, although numbers are feared to be far higher, aid workers say. Witnesses that AFP has spoken to recount a wave of atrocities, including an orchestrated campaign of mass killings and rape.

The unrest has also taken on an ethnic dimension, pitting Kiir's Dinka tribe against the Nuer tribe, to which Machar belongs.

With fighting ongoing, badly overstretched UN bases in the capital Juba and across the country have been flooded with at least 45,000 civilians, some of whom have recounted an orchestrated campaign of brutal killings and rape by government forces.

"Mass extrajudicial killings, the targeting of individuals on the basis of their ethnicity and arbitrary detentions have been documented in recent days," Pillay said.

"There need to be clear statements and clear steps from all those in positions of political and military control that human rights violations will not be tolerated and those responsible will be brought to justice."

UN chief Ban Ki-moon has asked the Security Council to nearly double the size of the UN mission in the country.

Two UN peacekeepers from India were killed last week when gunman stormed a UN compound in the restive Jonglei state where civilians were sheltering.