Thousands dead in South Sudan violence, UN says
UN Secretary General Ban Ki-moon: "The world is watching" South Sudan
Thousands of people must have been killed in the past week of violence in South Sudan, the top UN humanitarian co-ordinator there has told the BBC.
Toby Lanzer, who is in Bentiu in northern Unity state, said it had been "a devastating week for South Sudan".
The UN Security Council on Tuesday voted unanimously to almost double the number of peacekeepers to 12,500.
Earlier President Salva Kiir said his forces had recaptured the key town of Bor days after it was seized by rebels.
The rebels are led by Riek Machar, of the ethnic Nuer, who has been battling President Kiir, of the Dinka.
The UN also said on Tuesday that it had reports of at least three mass graves.
One was in Bentiu in the north, and two in the capital, Juba.
In a Christmas message, Mr Kiir said "innocent people have been wantonly killed", adding: "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."
'Palpable fear'Mr Lanzer told the BBC's Newshour programme: "I think it's undeniable at this stage that there must have been thousands of people who have lost their lives.
"When I've looked at the hospitals in key towns and I've looked at the hospitals in the capital itself, the range of injuries, this is no longer a situation where we can merely say it's hundreds of people who've lost their lives."
Mr Lanzer also said that the number of people seeking shelter from the fighting was "tens of thousands if not hundreds of thousands".
He said that the tensions between different communities in South Sudan was even evident within a UN base he had just visited where some 7,500 people are seeking protection.
Troops loyal to Mr Kiir had moved quickly from Juba to retake Bor The UN High Commissioner for Human Rights, Navi Pillay, said: "There is a palpable fear among civilians of both Dinka and Nuer backgrounds that they will be killed on the basis of their ethnicity."
The UN says at least 80,000 people have been displaced by the South Sudan crisis - about half of them seeking shelter with the UN.
Late on Tuesday the UN Security Council voted to increase its peacekeepers from 7,000 to 12,500, and its international police force from 900 to 1,323.
The BBC's James Copnall explains the fighting gripping the world's newest state, South Sudan - in 60 seconds
It authorised temporary transfers from missions in DR Congo, Darfur, Abyei, Ivory Coast and Liberia.
The council called for "an immediate cessation of hostilities and the immediate opening of a dialogue".
'Political solution'President Kiir earlier told journalists in Juba that his forces had "retaken Bor and are now clearing whatever forces that are remaining there".
Bor, 200km (125 miles) north of Juba, had been one of the major successes for the rebels. They are believed to still be in control of Bentiu.
Salva Kiir has agreed to talks "without preconditions" 
Riek Machar says he has formed a delegation for talks in Ethiopia Radio Tamazuj said government troops of the Sudan People's Liberation Army had launched attacks on positions held by Nuer commander and army defector Peter Gadet on Tuesday afternoon in Bor.
Mr Kiir said there was also fighting in the town of Malakal and that his troops were "ready to capture Bentiu".
President Kiir and Mr Machar have both said they are willing to talk.
However, Mr Machar has said his detained political allies must first be freed, while Mr Kiir says there should be no preconditions.
UN Secretary General Ban Ki-moon on Tuesday said there could be "no military solution to this conflict. This is a political crisis which requires a peaceful, political solution".
Mr Kiir has accused Mr Machar, who he sacked in July, of plotting a coup. Mr Machar denies he is trying to seize power.
Sudan suffered a 22-year civil war that left more than a million people dead before the South became independent in 2011.
Fighting erupted in the South Sudan capital, Juba, in mid-December. It followed a power struggle between President Salva Kiir, a Dinka, and his Nuer ex-deputy Riek Machar. The fear is that the rivalry will spark a widespread ethnic conflict. According to OCHA, 81,000 people have been forced from their homes. 
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam. 
Both Sudan and the South are reliant on oil revenue, which accounts for 98% of South Sudan's budget. They have fiercely disagreed over how to divide the oil wealth of the former united state - at one time production was shutdown for more than a year. Some 75% of the oil lies in the South but all the pipelines run north 
The two Sudans are very different geographically. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest. 
After gaining independence in 2011, South Sudan is the world's newest country - and one of its poorest. Figures from 2010 show some 69% of households now have access to clean water - up from 48% in 2006. However, just 2% of households have water on the premises. 
Just 29% of children attend primary school in South Sudan - however this is also an improvement on the 16% recorded in 2006. About 32% of primary-age boys attend, while just 25% of girls do. Overall, 64% of children who begin primary school reach the last grade. 
Almost 28% of children under the age of five in South Sudan are moderately or severely underweight - this compares with the 33% recorded in 2006. Unity state has the highest proportion of children suffering malnourishment (46%), while Central Equatoria has the lowest (17%). 
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