By Evelyn Leopold

UNITED NATIONS, Sept 1 (Reuters) - Malnutrition is on the rise in Darfur as a surge in violence prevents aid workers from reaching more people in need, a senior U.N. official said.

Eighteen spot surveys by U.N. agencies and independent humanitarian groups in three Darfur provinces indicated the emergency threshold of 15 per cent of the population suffering from malnutrition had increased to more than 17 percent in some areas, the U.N. deputy emergency relief coordinator, Margareta Wahlstrom, said on Friday.

Some of the malnutrition is attributed to lack of water and sanitation conditions as well as changing weather in Sudan's western province where African rebels rose up against the Arab-dominated central government four years ago, accusing it of neglect. Wahlstrom said the number of people forced to flee their homes was also a major factor.

"This is the first time we see the potential for a deterioration," since 2004 when a major relief effort began, she told a news conference.

Humanitarian efforts have also been hard hit and relief aid convoys have often been attacked.

"Aid workers are withdrawing temporarily or permanently from certain areas because it is impossible to work there," Wahlstrom said. "The violence directed against them has happened on 24 occasions this year alone."

Fighting involving pro-government militia and a myriad of rebel groups as well as bandits has displaced 55,000 people in the last three months and 250,000 since beginning of the year, U.N. figures show.

More than 2.2 million people in Darfur have been thrown out of their homes in the past four years and 4 million are dependent on aid out of a population of 6.4 million.

"A credible cease-fire and controlling the lawlessness in Darfur are really the two bottom lines that need to be sustained," Wahlstrom said. Otherwise the expectation that people would return to their homes "safely and voluntarily is not a realistic proposition."

The main hope is that once a new African Union-United Nations peacekeeping force of up to 26,000 troops and police arrives next year, some order could be restored, Wahlstrom said. "This is the urgent thing that we see ahead of us."

She said U.N. Secretary-General Ban Ki-moon would raise humanitarian concerns in Darfur when he sees Sudanese President Omar al-Bashir in Khartoum next week.

Still, humanitarian workers from nongovernmental groups have been apprehensive that Ban was giving too much priority to climate change as a cause of the Darfur conflict where desertification has shrunk farm lands and grazing pastures. Aid officials say the environment should not detract from pressure to get a cease-fire and establish law and order.

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