Sudanese troops barred the U.N. humanitarian chief on Saturday from visiting Darfur's most violence-plagued refugee camp during his first trip to the war-torn region.

The convoy carrying John Holmes was halted at a checkpoint about a mile outside the Kassab refugee camp, and he was told he did not have the proper papers to visit the site.

"I'm frustrated, annoyed, but it's not atypical of what happens here," Holmes told journalists traveling with him. He said his trip had obtained all the necessary clearances from Khartoum.

"This is rather typical of the kind of problem people are encountering in this kind of area. But it is interesting to see it in practice," he said.

The soldiers at the checkpoint briefly prevented a car carrying journalists from leaving after Holmes turned back from the site. The journalists were only allowed to leave after the troops took a videotape from a U.N. television cameraman.

A day earlier, Holmes had said he pressed Sudanese officials to grant better access to aid workers trying to help Darfurians amid widespread complaints that humanitarian groups face constant obstacles from the authorities in reaching victims of the conflict.

Darfur is the scene of the biggest humanitarian effort in the world, with some 4 million people in full or partial need for outside aid in a bloody conflict between Darfur rebels, the government and the pro-government janjaweed. More than 200,000 people have been killed and more than 2 million displaced in four years of fighting, and the Arab janjaweed are accused of widespread atrocities against ethnic African civilians.

Kassab, home to more than 25,000 refugees, has seen the highest level of rapes and other attacks against its residents. The camp is located in a region under tight control of the janjaweed and government forces, near the town of Kutum, 60 kilometers (40 miles) northwest of El Fasher, the North Darfur capital.

In February, the Sudanese government denied visas to a high-level mission of the U.N. Human Rights Council, mandated to report on the situation in Darfur.

The mission, led by U.S. Nobel laureate Jody Williams, produced its report outside the country, concluding that the government was responsible for orchestrating militia attacks against civilians in this beleagured Sudan region.