Sudan has barred the United Nations' top humanitarian official from visiting Darfur, raising fears that the government is trying to obscure the current extent of the violence and hardship in the region.

Jan Egeland, the UN Under-Secretary for Humanitarian Affairs, said his aircraft was turned away from the southern Sudanese town of Juba on Sunday and that he was forced to book a seat on a commercial flight from Uganda before being told that he was not welcome in the country.

"I’ve been barred from going to south Darfur, west Darfur and also I have been told that I am not welcome in Khartoum," Mr Egeland told Reuters news agency. "I think it is because they don’t want me to see how bad it is in Darfur."

Endemic violence, perpetrated by government-funded militias, is the greatest obstacle to the UN mission in Darfur, a region in the west of Sudan, as large as France, where more than three million people need aid to survive. An ethnic war, between Arab fighters and black African Sudanese farmers, broke out in 2003 and has killed more than 300,000 people.

Mr Egeland said his difficulties in reaching Darfur were evidence of the strains under which the UN and other aid agencies are working.

"This is symptomatic of the everyday problems my colleagues face in Darfur, trying to feed nearly three million Darfuris to whom we are a lifeline," he said.

The United Nations Mission in Sudan (UNMIS) said it "regrets" the government’s decision not to allow Egeland into Darfur and Khartoum as part of his five-day visit to assess the humanitarian relief operation in South Sudan and Darfur. Sudanese government officials were not available for comment.

In what is believed to be an unrelated development, the Sudanese government also refused to extend the mandate of the Norwegian Refugee Council, which runs the Kalma refugee camp in Darfur and shelters more than 100,000 people.

Astrid Sehl, a spokeswoman for the charity, said it had been negotiating with local authorities for two months before being told their told their contract, due to expire tomorrow, would not be extended.

"We don't know why, they're not telling us why," she told Times Online. "We're going to have to leave 100,000 extremely vulnerable people. This has huge humanitarian consequences."

According to Ms Sehl the security situation has deteriorated in Darfur in recent weeks, with the increasingly frequently attacks by the Janjaweed, Arab militias believed to be funded by the government.

She said Sudanese authorities were also becoming "quite creative" in the use of paperwork and bureaucratic measures to make the running of the aid programme more difficult. Petrol has been rationed and some aid workers have been given three-day visas, forcing them to leave the country twice a week to have them renewed.

The violence in Darfur started in early 2003 after local farmers rebelled against the Sudanese government, claiming that Khartoum discriminated against Sudanese of African origin in favour of ethnic Arabs.

Vicious repression followed, and the government is accused of directing Arab militias in a campaign of rape and widespread killing against the ethnic Africans. More than 300,000 people are thought to have died of disease and hunger since the fighting began, and a further 2 million have fled their homes.

Around 2,300 peace-keeping troops, sent by the African Union, are in Sudan, keeping a shaky ceasefire that has been in place since April 2004. Last year, the International Criminal Court launched an investigation of war crimes committed in the region. The US government has already described the disaster as "genocide".

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