Thousands are believed to have been killed in more than a week of violence, with reports of bodies piled in mass graves amid 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.
Government forces celebrated late yesterday the recapture of Bor from forces loyal to Machar after the army stormed the strategic town, but battles raged elsewhere including Malakal, capital of oil-producing Upper Nile state.
Information Minister Michael Makwei denied rebel claims they had seized it.
"There is fighting now in Malakal since morning between the government forces and the rebels," Makwei told AFP. "It is not true that the rebels have taken over."
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.
The UN humanitarian chief in the country, Toby Lanzer, said yesterday 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 world's youngest 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.
Around 15 bodies were found in one site in Bentiu, with another 20 bodies at a nearby river.
In Juba, the UN mission was more cautious, confirming the 15 killed but saying it was still "investigations reports of such atrocities."

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.
"There are now people who are targeting others because of their tribal affiliation," Kiir said in a Christmas message to the country, where the population is roughly divided between Christians, Muslims and traditional beliefs. "It will only lead to one thing and that is to turn this new nation into chaos."
Government forces have also said they are preparing to take back the town of Bentiu, capital of South Sudan's most important oil region Unity state, now in the hands of a powerful army commander who mutinied last week to join Machar.


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