The UN's new emergency relief coordinator John Holmes has been turned away from a camp in Darfur for those fleeing the Sudanese conflict.
John Holmes' tour includes Sudan's neighbours
The UN envoy was refused entry by Sudanese soldiers to Kassab camp in northern Darfur, says the BBC's Karen Allen, who is travelling with him.
In the past six months the BBC has reported on mass rapes of women and young girls at the camp.
Mr Holmes is on a tour of Sudan, Chad and the Central African Republic.
It was the former British diplomat's first visit to Darfur since taking over from Jan Egeland as UN undersecretary-general for humanitarian affairs on 1 March.
The Kassab camp is in northern Darfur, the region of western Sudan in the grip of a conflict which has seen 200,000 people killed and more than 2.5 million people displaced.
Within hours of arriving in Darfur, Mr Holmes was stopped at a checkpoint. His convoy was sent back and television groups covering the visit had their video tapes confiscated, our correspondent says.
The UN envoy told journalists travelling with him that the soldiers' actions illustrated the difficulties the UN is facing working in the area.
The Kassab camp, a few miles outside Kutum, is sanctuary to more than 20,000 people.
Our correspondent says that the camp has increasingly found itself cut off from the humanitarian community, which, she says, has effectively been forced out of the area.
Last week, Mr Holmes held talks with senior Sudanese officials in Khartoum and stressed the need for the government not to interfere with the humanitarian work, according to reports.
Aid agencies are trying to help some two-and-a-half million people who have fled from their homes after attacks from pro-government militia - the Janjaweed.
Our correspondent says that the fact that humanitarian access is being to denied to one of the UN's most senior envoys will do little to ease the diplomatic pressure to impose tough sanctions on Sudan.
After leaving Sudan, the UN envoy will travel to Chad and the Central African Republic, countries where tens of thousands of refugees from Darfur have fled, to escape the violence.