The UN has accused forces of President Salva Kiir and former vice president Riek Machar of carrying out atrocities in the conflict that started on December 25.

Amid mounting reports of mass killings in several towns, UN assistant secretary general Ivan Simonovic said on Thursday that 92 UN investigators are in the country and a first report would be released in about two weeks.

Speaking after a visit to the Unity state capital of Bentiu, Simonovic said it had been left a "ghost town" by successive raids by the rival forces. Nearly all the 40,000 inhabitants have fled.

"When there is change of control over Bentiu each time the group that was taking control was involved in human rights violations including killings of civilians," Simonovic was quoted as saying by the UN mission in South Sudan.

"We could see about 15 to 20 decomposing bodies just by the street," he added. "Obviously civilians are being tied before being killed."

Aid groups say up to 10,000 people have been killed in the battles and UN leader Ban Ki-moon has warned that both sides will be held "accountable".

Human Rights Watch said in a report released Thursday that it had interviewed witnesses to abuses in the capital Juba and Bor.

It said there had been "widespread killings" of ethnic Nuer men by Kiir's armed forces in Juba, including a massacre of between 200 and 300 men in the Gudele neighborhood on December 16.

The UN said it has stepped up security, including searches for weapons, at its compounds around the country where more than 65,000 people are estimated to have taken refuge.

BA/BA