Food and other basic relief is not reaching thousands in the war-torn Darfur region of Sudan, despite what the United Nations calls the world's biggest humanitarian effort.

Over a dozen aid workers have also been slain in recent months, and spiraling violence has forced many to pull out. Seventy-four World Food Program vehicles have been attacked and one driver has been killed since a peace treaty was signed in May between Khartoum and one of several rebel factions in Darfur. Other rebels rejected the deal.

Violence has been increasing and last month, in the worst looting yet, Arab tribal fighters known as janjaweed ripped apart a WFP warehouse and took 800 tons of food in the rebel stronghold of Bir Maza as government forces assaulted the town.

For more coverage on the crisis in Darfur and news on Africa, click here.

Over 200 U.N. and aid workers have had to leave remote outposts and refugee camps and some of the region's main towns — like the North Darfur capital of El Fasher, which last week was also looted by janjaweed.

Meanwhile, some 200 World Food Program trucks are being blocked by the government from reaching Darfur, said Kenro Oshidari, the Sudan director for the U.N. agency.

Janjaweed are not the only dangers. Three water engineers working with the U.N. Children's Fund were killed in June by refugees who thought they came to poison a well rather than fix it. Nine others were abducted in October and five are still being held, said UNICEF spokesman Edward Carwardine.

"Security is our most serious impediment throughout Darfur," he said.

The U.N. has called the Darfur conflict the world's worst humanitarian crisis. More than 200,000 people have been killed and more than 2.5 million driven from their homes in the three-year fight between the government and ethnic African rebels. The government is accused of unleashing the janjaweed to help put down the revolt, and the militia is accused of widespread atrocities against civilians.

The U.N. Human Rights Council held an emergency meeting Tuesday in Geneva to assess just how bad the crisis has become in Darfur.

"Food security is one of the most basic human rights, and it's constantly being challenged in Darfur," said Oshidari.

The WFP is the sole source of food for some 1.8 million people in Darfur, who without the U.N.'s help would starve because they fear marauding militias will kill or rape them if they leave the refugee camps to cultivate their fields. The WFP provides part of the dietary needs for nearly a million more people.

But it now cannot reach some 100,000 others — a number that fluctuates widely as lines of combat change — who are in desperate need, leaving them to rely on their own resources to find food. A few months ago, as many as 470,000 people were out of reach.

For more news on Darfur and Africa, click here.

Nearly 1 million tons of food have been delivered to Darfur, at a cost of more than $1 billion dollars since April 2004. Some 15,000 Sudanese and international aid workers have been mobilized for the effort, which has created the longest supply line in Africa, with trucks going 1,800 miles — a third on unpaved roads — from a Red Sea port to the West Darfur town of El Geneina near the Chadian border.

WFP officials say they have brought the malnutrition rate below emergency levels in Darfur since 2005.

But two years ago, the WFP could freely access all of Darfur, a vast, landlocked region of western Sudan that is nearly the size of Texas.

"Now we have to fly by helicopter" to many locations because of dangerous roads, Oshidari said.

The magnitude of the relief can be measured at the WFP's transit warehouse, a set of industrial-sized buildings on the outskirts of Khartoum.

Early Tuesday morning, workers busily unloaded 100 pound bags of rice from massive trailer trucks, each of which can carry more than 80 tons of food. The supplies were to be put on smaller trucks that can navigate the unpaved roads out to Darfur.

"Today isn't a busy day, we only have 1,000 tons to handle," said Lemma Bayissa, a WFP logistician. "At times, we've had to work until midnight to get all the bags through," he said, pointing at the sacks from the US Agency for International Development — which provides half of the food for Darfur.

Aid agencies warn yet another peril could be looming: donor fatigue.

The WFP had to reduce food rations this year because it was lacking cash. Though Oshidari is confident international donors will provide most of the $685 million needed for 2007, he wonders what will come next.

He fears the crisis could just drag on, with aid workers in Darfur barely helping people survive.

"A political solution has to be found," he said. "Or donors will tire."

For more news on Darfur and Africa, click here.