NAIROBI, Kenya — Child malnutrition rates have increased sharply in Darfur, even though it is home to the world’s largest aid operation, according to a new United Nations report.

Mustafa Ozer/Agence France-Presse - Getty Images A Turkish doctor examines a severely malnourished refugee in Darfur.

The report showed that 16.1 percent of children affected by the conflict in Darfur, a vast, turbulent region in western Sudan, are acutely malnourished, compared with 12.9 percent last year. For the first time since 2004, the malnutrition rate, a gauge of the population’s overall distress, has crossed what United Nations officials consider to be the emergency threshold.

Just as important, the increase has occurred despite the efforts of more than 13,000 relief workers in Darfur, who work for 13 United Nations agencies and some 80 private aid groups, and draw from an annual aid budget of about a billion dollars. Aid officials said they were concerned that even with all these resources, the condition of the people in Darfur seemed to be getting worse.

“This is a big deal,” said Jean Rigal, the head of a branch of Doctors Without Borders in Sudan. “The system is not working as expected.”

“Dr. Rigal said he was not exactly sure why child malnutrition rates were rising. But he cited more insecurity, restricted access for relief workers and a fresh round of displacements because of tribal fighting.

“There are many hypotheses,” he said.

The report seems to confirm what aid officials in Darfur have been saying for much of the past year: that the increasingly chaotic security situation, both inside the enormous camps of displaced people and in the desiccated rural areas that are very difficult to reach even in the best of times, has gotten to the point that it is hampering the delivery of much needed emergency food.

“The United Nations has been sounding the alarm about the deterioration of the nutritional situation in Darfur for months,” said Stephanie Bunker, a spokeswoman for the United Nations’ humanitarian operations.

She said that attacks on aid workers were up 150 percent and that “civilians continue to be displaced as a result of attacks from all sides, with almost 290,000 displaced in Darfur this year alone. Many camps for the displaced can no longer absorb new arrivals, and tensions are rising.”

Sudanese officials said that the United Nations was exaggerating the problems, and that life in Darfur had actually gotten better recently.

Stuart Price/African Union Mission in the Sudan, via Reuters A spokeswoman for the United Nations’ aid operations said that attacks on Darfur aid workers were up 150 percent this year.

Still, United Nations officials say the number of zones they could not reach has steadily increased this year, because of the attacks on aid workers and food convoys. Much of this violence seems to be a result of the fragmentation of the conflict, with rebel groups splintering into warring factions and formerly allied militias turning on one another.

To counter this, the United Nations and the African Union are trying to send in an expanded, joint peacekeeping force. But that deployment has been delayed by bureaucratic battles with the Sudanese government and the reluctance of developed countries to supply high-tech equipment, like helicopters.

As a result, people in Darfur are beginning to lose hope, and that may be another factor taking a toll on their health, several aid officials said.

“There is a psychological effect here,” said one aid official in Sudan who did not want to be identified because he feared reprisals from the Sudanese government. “These people have been in these camps for years now, and the energy that was around a few years ago and the hopes that this situation might be over soon and people could go home — all that’s gone now.”

He said that depression could affect how mothers care for their children, and that the overall malaise in the camps would make poor health conditions worse.

Darfur has been a humanitarian crisis since 2003, when rebels frustrated by a long history of marginalization attacked government forces. The government responded by arming tribal militias to wipe out the rebels and the civilians supporting them. Villages were burned, and countless women were raped; more than 200,000 people have died, according to conservative estimates.

From the beginning, the United Nations has used child malnutrition rates to assess the direness of the situation in Darfur. In 2004, when the conflict was raging, with waves of marauding horsemen sweeping the countryside, the acute malnutrition rate among the conflict-affected children was 21.8 percent. Then, in 2005, as hundreds of millions of dollars of resources were poured into the region, that rate dropped to 11.9 percent. It rose slightly to 12.9 percent last year. The United Nations considers 15 percent to be the emergency threshold.

The new United Nations report is based on information collected in August and September from thousands of Darfurians affected by the conflict, including those living in squalid camps (the United Nations estimates roughly 2.2 million people have been displaced by fighting). The report cited “consistently poor infant and young child feeding practices” and a “deterioration in the overall food security situation.”

The report also showed that the percentage of Darfurians growing their own crops had decreased this year. The people surveyed said that insecurity and a lack of access to their farms were the main reasons, though Sudanese officials have hypothesized that some Darfurians may have simply grown dependent on food aid and chosen to stop farming.

Malnutrition was highest among young children, between 6 months and 29 months old, and in North Darfur State, which is sparsely populated and very dry.

The report highlighted improvements in access to clean water and more people using latrines. And the rate of severe acute malnutrition — the worst cases — remained the same as last year at 1.9 percent. Child malnutrition is measured by using a ratio of weight versus height, and the severity is determined by how much the ratio deviates from the standards for healthy children.

Malnutrition rates are a highly sensitive subject in Sudan, and Sudanese government officials have objected to some of these findings, taking issue with the survey methodology and the overall characterization of the problem.

“It’s true, there is a gap of food in Darfur, and the conflict is not settled yet,” said Rabie A. Atti, a government spokesman. “But from our information, the situation is better now than before.”