More than two million have been displaced by the conflict
An estimated 300,000 people may have died as a result of the Darfur conflict, the UN head of humanitarian affairs John Holmes says.
That is an increase of 50% from the previous figure of 200,000 killed in the five years of fighting in the region in western Sudan.
Mr Holmes gave the revised total to a meeting of the United Nations Security Council in New York.
Sudan disputes the figure, saying 10,000 are now known to have died.
The previous figure of 200,000 came from a 2006 study by the World Health Organisation.
It included those killed in the fighting itself as well as people who died from disease and malnutrition because of the conflict.
The 2006 figure "must be much higher now - perhaps as much as half again," Mr Holmes said.
He said the new total was an extrapolation from the previous figure and was not based on a new study.
Speaking later to reporters, Mr Holmes added: "I am not trying to suggest this is a very scientifically-based figure. It is extrapolated from the 2006 figure, it is not new research."
Mr Holmes said: "We continue to see the goal posts receding to the point where peace in Darfur seems further away today than ever."
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Mr Holmes' comments were hotly disputed by the Sudanese ambassador to the UN, Abdul Mahmoud Abdel-Halim, who accused him of exaggerating the figure.
"These remarks by Holmes are not helpful, are not correct, are not credible," Mr Abdel-Halim told the Reuters news agency.
"He should tell us who made that study, who commissioned it and how was it done."
Mr Abdel-Halim put the number who died at 10,000, which is 1,000 higher than the Khartoum government's previous estimate.
The Sudanese number only comprises those who have died in combat, he said.
Eric Reeves, the American author of a book on Darfur, "A long day's dying" says the figure may be even higher than that estimated by Mr Holmes.
"We know that the figure has been far too low, for far too long," Mr Reeves told the BBC.
"Mr Holmes said that 300,000 was a conservative figure. In my own view based on many mortality assessments, looking at all the data that I can put my hands on the figure is likely to be closer to 500,000," he said.
He also said that the government in Khartoum was refusing to allow mortality studies to be conducted in Darfur.
The Security Council was also told that the joint UN-African Union peacekeeping force for Darfur is unlikely to be up to full strength this year.
The joint UN-AU special representative for Darfur, Rodolphe Adada, said troop levels in the UNAMID mission he leads "is at less than 40% of its mandated level of 19,555 and it is very unlikely to achieve full-operating capability before 2009".
After the meeting, he suggested the force could reach 80% of its strength at the end of this year as attempts were made to accelerate deployment.
The conflict in Darfur in western Sudan began in 2003 after rebel groups began attacking government targets, saying their communities were being discriminated against in favour of Arabs.
In retaliation, the government launched a military and police campaign in Darfur, prompting 2m people to leave their homes.
Arab Janjaweed militia are accused of following up on government raids on villages with a campaign of murder and rape.
The Sudanese government denies links to the Janjaweed, but admits establishing "self-defence militias". It says the problems have been exaggerated.
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