Thousands of South Sudanese have been killed in more than a week of violence, with reports of bodies piled in mass graves, the UN said as it moved to nearly double its peacekeepers there.

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 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," 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.

US Secretary of State John Kerry called both men and urged them to "accept a cessation of hostilities and begin mediated political talks," the State Department said.

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.

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 about 20 ethnic Dinka civilians sheltering there.

UN vote on extra troops

Fighting has spread to half the country's 10 states, the United Nations said, with hundreds of thousands fleeing to the countryside and others flooding UN bases seeking shelter.

Pillay's spokeswoman told AFP that a UN official had visited a mass killing site Monday in Bentiu, the capital of the oil-rich Unity state, and counted at least 34 bodies.

Rebel fighters are also reported to have committed atrocities in areas they control.

Late on Tuesday, the UN Security Council voted to send nearly 6,000 extra soldiers and police to South Sudan, nearly doubling the UNMISS force to 12,500 troops and 1,323 civilian police.

But UN chief Ban Ki-moon, who requested the reinforcements, warned the force "will not be able to protect every civilian in need in South Sudan."

"This is a political crisis which requires a peaceful, political solution," he said.

He called on the rival leaders to "save your proud and newly independent country."

The US military deployed a "platoon-sized" Marine contingent to neighbouring Uganda to protect US citizens and facilities in South Sudan and prepare for possible further evacuations of Americans, a spokesman said without specifying the number of troops involved.

Nearly 100 US troops were already on the ground in South Sudan, including a contingent reinforcing security at the American embassy.

The Pentagon also deployed a roughly 150-strong special Marine Corps unit Monday to Djibouti, along with cargo planes and helicopters.

On Saturday, a US evacuation operation had to be called off when American aircraft came under fire, with four troops wounded.

South Sudan gained independence in July 2011 and is still the youngest country in the world, born out of a bloody decades-long struggle for independence from Sudan.

It remains a fragile state with deep ethnic divisions.

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.

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