The top UN humanitarian chief in the country Toby Lanzer said today 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 today 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.
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.
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Witnesses that AFP has spoken to recount a wave of atrocities, including an orchestrated campaign of ethnic mass killings and rape.
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 today 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.