"It is a hepatitis E outbreak, which shows the need to improve sanitation and access to potable water. We must sustain the surveillance system to detect early stages of all kinds of outbreaks," WHO spokeswoman Fadela Chaib said.

Hepatitis E, caught from dirty water or food, typically strikes people between the ages of 15 and 40 and kills five per cent of victims. It is especially dangerous for pregnant women.

Aid workers have warned of cholera, dysentery and malaria outbreaks in Darfur, where more than one million people have been displaced by a conflict that began early last year.

"This is just the beginning of the rainy season. Soon the mosquitoes (which carry malaria) will have an environment to breed," said Abiy Tamrat, an MSF doctor who just returned from West Darfur. "We have predicted that cholera, measles and other epidemics will start with the rainy season. That still holds true."

He said the hepatitis outbreak confirmed the need for better measures to provide the displaced with safe drinking water and soap, and collect and dispose of garbage and human waste.

"If the conditions are right, in absolute numbers it (hepatitis E) could potentially cause huge mortality if the disease spreads continually and there is no intervention," Tamrat said.

The WHO said the area around the Kalma and Musei camps in southern Darfur had already been sprayed against mosquitoes, and that spraying around camps in the north would begin this week and in west Darfur on August 20.

Chaib said a first round of cholera vaccination was completed in late July at Kalma camp, and a second round of distribution of the oral vaccine began at the weekend.

The U.N.'s World Food Program has been flying in food supplies, but Tamrat said surveys indicated that between two and five percent of children under five in Darfur were severely malnourished, and up to 20 percent had less acute malnutrition.

"If food is not adequate before the rainy season, we have reason to believe it can get worse despite the air drops," he said.

International bodies have estimated that up to 50,000 people have already died as a result of the conflict.

Two rebel groups took up arms against the government in early 2003. Rebels accuse the Khartoum government of arming Arab militia known as Janjaweed to carry out ethnic cleansing. The government denies this and says the Janjaweed are outlaws.