By Opheera McDoom

KHARTOUM, June 22 (Reuters) - Darfur faces a food crisis this year as a result of a "perfect storm" of growing violence, overcrowding in refugee camps and a bad harvest, the United Nations said on Sunday.

The conflict in Darfur has prompted the world's largest humanitarian operation, which helps two-thirds of the region's 6 million population.

But this year, a confluence of problems in the western Sudanese region could cause a spike in malnutrition rates and health problems, United Nations officials said.

"Four years into a massive humanitarian operation ... this is the year I think where we are going to see a reduction in (health) indicators as a result of this perfect storm," Mike McDonough, head of the U.N. humanitarian coordination office, said.

Attacks on World Food Programme food convoys have forced a cut in rations to millions in Darfur by almost half since May, and government promises of escorts for aid trucks have not materialised.

"If we don't improve we simply cannot raise this ration in the future," WFP head Kenro Oshidari said at a joint U.N. agencies news conference. "The reality is that the trucks need to move every single day."

Ted Chaiban, head of the U.N. children's agency UNICEF, said without trucks moving and full rations, this year's traditional hunger gap as rains cut off roads and farms would be more severe than previous years.

He said a worse harvest this year had already led to prices of basic commodities doubling in Darfur's markets.

"We are in the middle of what is going to be a challenging hunger gap," he said.

"We are holding the line but we are concerned ... that if the food ration is not resumed in full, things will deteriorate."

With malnutrition rates already high, he said August and September, traditionally bad months for hunger, could see a massive hike in the numbers of malnourished Darfuris.

On reduced rations, Darfuris will be more vulnerable to disease.

There are few accurate malnutrition rates for Darfur because the government has until now refused to allow surveys to be released, saying they would be exaggerated by Western media and activists.

Chaiban said the government had finally agreed to release 11 withheld malnutrition surveys.

Previous unpublished surveys have indicated malnutrition rates above emergency levels in "pockets" of Darfur, U.N. officials have said.

Mortality surveys have been blocked by Khartoum for three years. Sudan says the U.N. estimates of between 200,000 and 300,000 killed in Darfur are exaggerated and says just 10,000 have died. (Editing by Giles Elgood)

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