The new U.N. humanitarian chief warned Sunday that humanitarian efforts in Darfur could collapse if the situation deteriorates and aid workers are prevented from doing their work.

John Holmes spoke while visiting a refugee camp on the outskirts of the town of El-Fasher in Darfur, on his first tour of the troubled region since becoming the U.N.'s top humanitarian official. Some 45,000 people have taken refuge in the camp from the region's spiraling violence.

"This humanitarian effort is fragile," he said. "If the situation deteriorates, it could collapse."

Holmes, who met with delegates of international aid groups on Saturday and Sunday during his two-day visit to Darfur, said problems such as obstruction from Sudan's government and insecurity on the ground have created an environment where "morale is fragile" and could push aid workers to pull out.

"The risk is high," he said. "It is not imminent, but if things deteriorate, people may not want to maintain their efforts."

Because humanitarian needs are "so huge," Holmes said that aid groups obviously do not want to pull out of Darfur.

"But if there is a big incident, for instance, people could be fed up," he added.

In need of aid are some 4 million people in Darfur who the U.N. says have been caught in the midst of fighting between 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 janjaweed Arab militias are accused of widespread atrocities against ethnic African civilians.

The U.N. says the conflict has chased another 86,000 people from their homes so far this year and blames the vast majority of these new refugees on violence perpetrated by central Sudanese government forces or their allied janjaweed militias.

Former rebels aligned with Minni Minawi, who signed a peace deal with the government last May and has now joined forces with Sudan's central government, are also increasingly blamed for the violence.

Es Sallam, the refugee camp visited by Holmes, is one of three camps near El Fasher. It is overspilling with people, and aid workers are currently negotiating space for a fourth camp to meet the incoming flow of refugees.

Holmes said people in the camp were not starving and health conditions seemed decent.

"This shows the enormous humanitarian effort that has been made for three years," he said, referring to the international aid effort in Darfur, which is the largest in the world with over US $1 billion spent and some 14,000 aid workers in the region.

Holmes had been turned away the day before by Sudanese military police when he tried to visit a refugee camp. On Sunday, the governor of north Darfur extended apologies to Holmes for the incident.

Holmes said he accepted the apology but would nonetheless raise the issue with Sudanese officials because it illustrated the near-constant problems faced by relief workers trying to deliver aid to Darfur's population.