Thousands of South Sudanese have been killed in over a week of violence with reports of bodies piled in mass graves, the United Nations says, amid ongoing battles in the young nation threatening to slide into civil war.
The UN Security Council has voted to increase their peacekeeping force in the country to 12,500 from 7,000.
The top UN humanitarian chief in the country Toby Lanzer said 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 country.
However, the government also celebrated 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.
In a Christmas message to the people, Kiir said that "innocent people have been wantonly killed", warning the violence risked spiralling 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," Kiir said in a statement.
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 reports of crimes against humanity will be investigated.
Machar said for the first time Tuesday that he was "ready" to accept Kiir's offer of talks, suggesting neighbouring Ethiopia as a neutral location.
"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.
UN peacekeepers had spent days bolstering fortifications ahead of the army assault, after militia gunmen last week stormed a UN compound in the Jonglei outpost of Akobo, killing two Indian soldiers and some 20 ethnic Dinka civilians sheltering there.
Fighting has spread to half of the young nation's 10 states, the United Nations said Tuesday, with hundreds of thousands fleeing to the countryside and UN bases flooded with others seeking shelter, prompting warnings of an imminent humanitarian disaster.
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 counted 14 bodies in the grave and 20 at a riverside nearby, but 74 ethnic Dinka soldiers are also missing feared dead, she said.
Rebel fighters are also reported to have committed atrocities in areas they control as the impoverished nation, which won independence from Sudan to much fanfare just two years ago, appeared to be sliding deeper into civil war.
President Kiir has accused Machar of starting the fighting by attempting a coup, while Machar says the president has exploited tensions within the army to carry out a purge.
Speaking from the relative safety of a UN base in Juba, two ethnic Nuer men alleged they were arrested by government soldiers along with an estimated 250 other men, herded into a police station in the capital Juba and then fired on.
"It was horrible, because to survive you had to cover yourself with the bodies of dead people, and... the bodies started to smell really bad," said one of the men, named Simon, who would give only his first name for fear of reprisals.
"We remained only 12 people. The rest were killed off," said Gatwech, another survivor and witness to the alleged massacre, who was also nursing several wounds and recounted similar details.
The government has denied being behind any ethnic violence.