The Sudanese security forces surrounded several camps in the war-torn region of Darfur (search) 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 (search) 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 the southern Darfur city of 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 from the Sherif camp, where they were taken.

"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.

The U.N. envoy, who is scheduled to report to the U.N. Security Council on Thursday on the situation in Darfur, confirmed that the "overall" security situation in the vast western region — which is the size of France — has deteriorated in the last few weeks.

While he blamed Sudanese forces in southern Darfur, Pronk left open the possibility that the forced transfer from El Geer was not carried out on instructions from the government in Khartoum.

In Washington, State Department press officer Tom Casey said the Bush administration was "outraged" by the reports.

"We call on the government of Sudan to cease any attempt to relocate people against their will and to allow immediate access to humanitarian workers," Casey said.

Sudan, however, denied any army or police forces were surrounding the camps. "There is no siege," Humanitarian Affairs Minister Ibrahim Hamid (search) told The Associated Press. "It is not true that the government was telling organizations to pull out of the area, and the areas are not besieged."

Hamid said angry Arab tribesmen gathered in the area after the kidnapping of 18 of their men by rebels in western Darfur. "The African Union has been alerted and they said they would bring those abducted out of the mountainous areas of Zaleinge," he said.

Abdulrahman Zuma, a Sudanese government spokesman at Darfur peace talks in Nigeria, said later Tuesday that 6,000 Arab militiamen had "invaded" the western camp of Zaleinge and freed the 18 hostages along with another 11 captives captured by rebels at an earlier date. He gave no further details.

The United Nations pulled 88 relief workers — most working for independent aid groups — from three western camps in the towns of Golu, Zaleinge and Nertetie.

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.

Attacks have uprooted 1.5 million of Darfur's people, and at least 70,000 have died, mostly through disease and hunger, according to the world body. The United Nations and aid groups have called Darfur the world's worst humanitarian crisis.