Khartoum, Sudan ? The Sudanese security forces surrounded several camps in the war-torn region of Darfur on Tuesday, relocated refugees against their will and denied access to humanitarian groups, the United Nations said. Sudan denied closing off the camps but said angry Arab tribesmen gathered in the area.

The U.N. World Food Program said several camps were surrounded, apparently in retaliation for the abduction of 18 Arabs by Darfur rebels, and that the world body was forced to pull 88 relief workers from other areas where there has been an upsurge in violence in recent days.

The World Food Program fears the government may start forcing people from the camps back to their home villages, where there is less protection from government-backed militias known as Janjaweed that have been attacking towns, said spokeswoman Christiane Berthiaume.

The camps, located near Nyala, were cut off “at 3 a.m. without any warning,” she said. “Agencies have been denied access to these camps since this morning.”

At least 160,000 refugees cannot be reached by road “because of insecurity,” Berthiaume said.

The top U.N. envoy to Sudan accused security forces in southern Darfur of forcing several thousand people who had taken refuge in the El Geer camp to move against their will in “flagrant violation” of international law.

Jan Pronk, speaking at the United Nations, demanded that all those rounded up and forced to leave the camp at 3 a.m. be returned immediately.

“It has to stop — not only in El Geer but as a policy everywhere,” Pronk said, demanding that the government keep its agreement with the United Nations barring the forced transfer of any internally displaced people.

Sudan’s government is accused of backing the Janjaweed in a campaign of violence — including rapes, killings and the burning of villages — to help put down a 19-month rebellion by non-Arab African groups. The government denies backing the militias.

The United Nations has called it the world’s worst humanitarian crisis.