Large areas of South Sudan remain out of the government's control amid fears the young nation was sliding toward civil war, though Juba said its forces had recaptured the strategically important town of Bor from rebels on Tuesday.
The UN humanitarian chief in the country, Toby Lanzer, said there was "absolutely no doubt in my mind that we're into the thousands" of dead, the first clear indication of the scale of the conflict engulfing the country.
Earlier, UN rights chief Navi Pillay said a mass grave had been found in the rebel-held town of Bentiu and cited reports of at least two more in 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, but aid workers have said the number killed was likely far higher.
Witnesses recount a wave of atrocities, including an orchestrated campaign of mass killings and rape.
In a Christmas message to the people, Kiir said that "innocent people have been wantonly killed," warning that the violence risked spiraling out of control.
"There are now people who are targeting others because of their tribal affiliation.... It will only lead to one thing and that is to turn this new nation into chaos," he added.
The unrest has taken on an ethnic dimension, pitting Kiir's Dinka tribe against Machar's Nuer.
Machar said he was ready to accept Kiir's offer of talks, following days of shuttle diplomacy by African nations and calls from Western powers for an end to the fighting.
"We want democratic, free and fair elections. We want Salva Kiir to call it a day," Machar said, listing his demands.
Machar's promise of talks came shortly before the army stormed Bor, 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 about 17,000 besieged civilians who fled to the UN peacekeeping compound for protection, severely stretching limited food and supplies.
BA/BA