Sudan has prevented the United Nations' top humanitarian official from visiting the troubled Darfur region.
Millions still rely on food aid and emergency relief
Jan Egeland told the BBC he thought the government did not want him to see the latest wave of "ethnic cleansing" against black Africans in South Darfur.
He said thousands of people had fled after 60 villages were attacked by pro-government Janjaweed militias.
More than two million people have sought refuge in huge camps following three years of such attacks.
The Sudanese government is trying to stop the UN from taking over the peacekeeping mission in Darfur from the under-funded African Union.
The United Nations Mission in Sudan has lodged a formal protest against the government's refusal to allow Mr Egeland's flight to land.
Sudanese foreign ministry spokesman Jamal Ibrahim said the government had asked Mr Egeland to delay his visit because it coincided with a holiday to mark the birthday of the Muslim Prophet Muhammad.
Jan Egeland is known for speaking his mind
He said that in the light of the Danish cartoons row, it would not be sensitive or safe for a Norwegian such as Mr Egeland to visit.
The BBC's Jonah Fisher in Sudan says Mr Egeland is known for his willingness to speak his mind and has been a strong critic of the government's role in Darfur's violence.
Mr Egeland said the rebel Sudan Liberation Army also had some responsibility for provoking the latest attacks in the Janana area.
He said the Sudanese government, guerrilla forces and ethnic militia groups were all responsible for the current instability in Darfur, which had put tens of thousands of civilians at risk.
He was due to meet aid workers and speak to people displaced by the conflict.
Mr Egeland also warned that hopes for peace in the south of the country were being damaged by continuing violence.
A peace deal last year officially ended 21 years of conflict between the northern Muslim government and rebels from the Christian and animist south.
On Saturday Mr Egeland visited Uganda, where he described the activities of rebels in the north of the country as "terrorism of the worst kind anywhere in the world".
Fear of attack
Ahead of his visit to Darfur, Mr Egeland, the UN's top humanitarian co-ordinator, said UN relief efforts were being undermined by poor security.
"We are being attacked and our humanitarian services disrupted all the time," he told AFP news agency.
In early 2004 he labelled Darfur the world's worst humanitarian crisis.
Millions of people had fled their homes as militia rampaged through villages, burning, killing and raping.
There are no official figures on how many died in what the US called a genocide, though estimates range from between 100,000 to 400,000.
Our correspondent says that almost two years on, the media spotlight has gone but the crisis is far from over.
Fearing militia attacks, two million people live in overcrowded camps totally dependent on international agencies for food and water.
The violence has less intensity but has become more complex, our correspondent adds.
New rebel movements have formed and existing ones have split and fought each other.
Pro-government militia continue to attack villages and camps with, according to many observers, the backing of the Sudanese army.
The government denies backing the Janjaweed and blames the violence on the rebels who took up arms three years ago.
Rebel movements from eastern Chad now use Darfur's anarchy as their base, creating insecurity in border areas.