The UN high commissioner for human rights has warned of "huge" violations against civilians in South Sudan, as she led a team visiting the country's capital, Juba.

Navi Pillay was accompanied on Tuesday by Adama Dieng, the UN's special adviser for the prevention of genocide, and met Barnaba Marial Benjamin, South Sudan's foreign minister, to discuss the crisis, the AP news agency reported.

As we say in Africa, when two elephants fight it's the grass that suffers. Navi Pillay, UN High Commissioner for Human Rights

"As we say in Africa, when two elephants fight it's the grass that suffers. Well, here I see civilians, ordinary people who are suffering and they are suffering huge human rights violations," Pillay said.

The visit comes after the UN Security Council expressed "horror" at the recent massacre of several hundred civilians in the city of Bentiu, which it blamed on rebel fighters.

On Monday, international mediators called on South Sudan's rebel leader to meet his rival, President Salva Kiir, to prevent an conflict fuelled by ethnicity turning into a genocide.

Alexander Rondos, an EU envoy, said such a face-to-face meeting between the rivals, which would be the first since fighting erupted in mid-December, was essential to end the "cycle of vengeance" and killings.

"The situation is now so combustible that it has all the ingredients of an all-out civil war in which the consequences could end up being genocidal," Rondos told the Reuters news agency.