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."
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.
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.
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".
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.
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.