Thousands of South Sudanese have been killed in over a week of violence with reports of bodies piled in mass graves, the UN says, amid ongoing battles threatening to slide into civil war.
The top UN humanitarian chief in the country, Toby Lanzer, said on Tuesday there was "absolutely no doubt in my mind that we're into the thousands" of dead, the first clear indication of the scale of conflict and ethnic violence engulfing the world's youngest nation.
However, the government also celebrated on Tuesday the important and strategic recapture of the key town of Bor after a nearly week-long rebel occupation, although large areas remain out of their control.
Earlier, UN rights chief Navi Pillay said a mass grave had been found in the rebel-held town of Bentiu, while there were "reportedly at least two other mass graves" in the capital Juba.
The grim discovery follows escalating battles between troops loyal to President Salva Kiir and those backing his rival Riek Machar, a former vice president who was sacked in July.
The official toll nationwide 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 ethnic mass killings and rape.
The unrest has taken on an ethnic dimension, pitting Kiir's Dinka tribe against the Nuer tribe to which Machar belongs.
UN chief Ban Ki-moon has warned warring factions that reports of crimes against humanity will be investigated, as well as asking the Security Council to nearly double the size of the UN mission in the country.
However, Machar said for the first time on Tuesday that he was ready to accept Kiir's offer of talks, suggesting neighbouring Ethiopia as a neutral location.
"We are ready for talks," he told Radio France Internationale (RFI), adding that he had spoken earlier in the day to US Secretary of State John Kerry and Ethiopia's Foreign Minster Tedros Adhanom.
"We want democratic free and fair elections. We want Salva Kiir to call it a day," Machar said, listing his demands, which follow days of shuttle diplomacy by African nations and calls from Western powers for fighting to stop.
Machar's promise of talks came shortly before the army stormed Bor town, which Information Minister Michael Makwei called a "gift of the government of South Sudan to the people".
Bor's capture, apparently without major resistance by the rebels, relieves some 17,000 besieged civilians who fled into the overstretched UN peacekeeping compound for protection, severely stretching limited food and supplies.
Pillay's spokeswoman told AFP that a UN official had on Monday visited a mass killing site in Bentiu, the capital of the oil-rich Unity State, and counted at least 34 bodies with dozens more feared dead.
"The UN official who visited saw 14 bodies in the grave and another 20 at a riverside nearby," but 74 ethnic Dinka soldiers are also missing feared dead, she said.
UN bases have been flooded with tens of thousands of civilians, including some 20,000 in the capital Juba.
UN chief Ban has recommended that the Security Council send 5500 more soldiers to reinforce the existing 7000 troops from the UN Mission in South Sudan. He also called for hundreds more police officers.
The Security Council is set to vote on the matter later on Tuesday.
