Kenyan President Uhuru Kenyatta and Ethiopian Prime Minister Hailemariam Desalegn were in the South Sudanese capital Juba on Thursday to attempt to mediate in the conflict.

Hundreds of people have been killed since last week, when a growing power struggle between President Kiir and his former vice president, Riek Machar, turned violent.

Fighting between troops loyal to either side became particularly intense in the Central Equatoria, Unity and Jonglei states, with hundreds of people killed and tens of thousands displaced.

"There are now people who are targeting others because of their tribal affiliation," Kiir said in a Christmas message. Kiir and Machar men are from different ethnic groups, which has exacerbated tensions.

Mass graves in capital

UN human rights chief Navi Pillay said a mass grave had been found in rebel-held Bentiu. She also cited reports of at least two more such graves in Juba.

Kiir, an ethnic Dinka - the largest group in South Sudan - has said he is willing to negotiate and enter talks unconditionally.

Machar - ethnically Nuer - who is in hiding, is demanding that Kiir step down, accusing him of being dictatorial. But he has also expressed an interest in talks, calling for any negotiations to be held in the Ethiopian capital Addis Ababa.

South Sudanese Information Minister Michael Makuei Leuth said the government has not yet established formal contact with Machar. "For us, we are not talking with him," Leuth said.

Machar has submitted a list of potential delegates, including the former Sudan People's Liberation Movement secretary general Pagan Amum Okiech. Other senior party leaders from the movement, currently imprisoned in Juba, are also on the list.

rc / bk (AP, dpa)