The wobbly bus chugged up through the hills, bound for Fadel Sesese Mohamed's village. He was feeling tired but optimistic after a long journey from Libya, where the 72-year-old tribal leader had participated in a peace conference last month aimed at ending Sudan's civil conflict.

The bus suddenly stopped, and a group of men armed with AK-47 rifles leaped on board, Mohamed recounted last week. They poked at the luggage and asked if any passengers had military ID cards. They pulled a group of men off the bus, punching one over and over and shouting, "Are you a soldier?"

Then the gunmen, who wore the green and brown netted head covers of a rebel group called the Sudanese Liberation Army, motioned at the bus driver to resume his journey. Most of the hostages have not been seen since.

There are contradictory reports about how many passengers were taken. The United Nations has said 18 were taken. The African Union said five of the hostages had been released. It is also not clear whether they were civilians or soldiers, and whether they were all Arabs or included some Africans.

But the incident, which alarmed the international aid community, has highlighted the growing number of attacks by African rebels. Until now most of the human rights abuses in the western region of Darfur have been blamed on pro-government Arab militiamen.

Officials of the United Nations and the African Union, which has sent a force of military observers into Darfur to monitor a shaky cease-fire, said the hostage-taking incident also shows that rebel groups -- not just the Arab militias known as Janjaweed -- must be pressured to uphold an agreement signed in Abuja, Nigeria, on Nov. 9.

In a report issued this week, Human Rights Watch strongly criticized the Khartoum government for fueling the conflict, but it also blamed rebel groups and urged the Sudanese Liberation Army to release all hostages.

"There's no security. There's no stability. This is the main problem," said Mohamed, a leader of the Fur tribe, waving his walking stick in agitation as he recounted the story of the hostage-taking at home here. He said he sympathized with the African rebels, but that in this case they had "made a mistake."

Within hours of the hostage-taking, Janjaweed militiamen threatened to attack this hillside town and a cluster of nearby refugee camps if the captives were not released. Foreign relief organizations quickly pulled 82 staffers out of the area. No attack took place, but the workers have not returned.

Meanwhile, tribal leaders and human rights groups said, the government responded to the crisis by arming local Janjaweed members. On Oct. 26, a squad of uniformed Janjaweed militiamen ambushed an African Union mission trying to retrieve the hostages, killing a Sudanese Liberation Army commander and four others, officials said.

In camps for displaced families surrounding Zalingei, food was distributed Saturday for the first time since the end of September. Highway banditry had cut off aid to the 82,000 people there, and the once-bustling town remains in limbo, with few buses traveling the hazardous roads and only helicopters able to reach the remote spot.

Tony P. Hall, the U.S. ambassador to the U.N. World Food Program, flew to Zalingei to tour the camps last week. Families living in tattered shelters told him that security was their main concern. On Friday, more than 50 women who ventured outside the camp to collect straw and firewood were stopped by militia forces and held for more than five hours, aid workers told Hall.

U.N. and aid officials said the African rebel groups, which enjoy support from African tribes in the area, are hurting the local populace as they resort to such tactics as ambushing food convoys, stealing aid trucks and taking hostages.

"The rebels are using the wrong instruments to make their points," Jan Pronk, the U.N. special representative to Sudan, said during a visit earlier this month to Nyala, the capital of South Darfur. "They have to stop. Otherwise they are blocking access to the very people they say they are protecting."

The Darfur conflict erupted in February 2003, after the Sudanese Liberation Army and another rebel group, the Justice and Equality Movement, attacked police stations and military outposts to protest what they called regional discrimination by the mostly Arab clique controlling Sudan's government.

Relief groups say the government responded by arming and supporting the Janjaweed, an Arabic word often translated as "devils on horseback," to crush the rebellion. Tens of thousands of people have died in the violence and more than 1.5 million have been uprooted.

Abdou Abdullah, a leader of the Sudanese Liberation Army and a member of the African Union's cease-fire commission, said the rebel group had never mistreated its hostages, most of whom were soldiers. He promised the group would stop taking hostages.

"We are serious about the peace deal," Abdullah said in a telephone interview from El Fasher. "Now we have to see that the government also keeps its promises and stops attacking us and our villages."

Last week, the rebel group turned 20 captured government soldiers and police over to the African Union mission.

In the fetid camps that stretch for miles outside Zalingei, people have barely enough to eat, and relief officials said the food situation could become desperate if security was not established.

Inside one camp this week, old men passed out from the relentless heat. Children flew kites made of plastic bags, and a few donkeys -- survivors of wartime livestock looting -- wore leather pouches filled with Koranic verses to guard against danger.

Khartouma Mohamed Abakar, 25, sat despondently in her shelter, too depressed to adjust her disheveled scarf or shoo the flies off her sleeping infant. Her husband was killed in a militia attack on their village, 20 miles north. Last week, when she went out to collect firewood, she said a strange man taunted and beat her, calling her a "rebel wife."

Abakar has one small bag of millet left over from the last food distribution to feed her children. She dreamily recalled better times, when the family had mutton, sugar cane, watermelon and cucumbers to eat. Now, she said, they are grateful for the gooey yellow porridge that keeps them alive.

A refugee camp near Zalingei, in the western region of Darfur, has been tense since an Arab militia threatened to retaliate for kidnappings by African rebels. "There's no security," says Fadel Sesese Mohamed, a 72-year-old tribal leader of the Fur tribe.