Sudan is violating international law and its agreements with the United Nations by forcing homeless people out of camps they had fled to in Darfur, UN Secretary-General Kofi Annan has said.
Mr Annan is also concerned by the deteriorating security situation in Darfur, where tensions have increased following the kidnapping of 18 hostages by the Sudan Liberation Army rebel group and the mobilisation of thousands of Arab militias in West and South Darfur, a spokesman said.
Mr Annan called on all sides to stop fighting, urged the rebels to release their hostages and appealed to the militias to stand down.
The incident in the camps involved several thousand refugees among some 20,000 homeless people near Nyala in South Darfur. Under agreements with the United Nations, Sudan cannot force them to return to their villages or other locations until they agree to go.
"I strongly urge the government to halt immediately all such relocation operations and to facilitate the return of the affected persons from the inappropriate sites to which they have been taken," Mr Annan said in a statement.
Mr Jan Pronk, appointed by Mr Annan to try to negotiate an end to the Darfur crisis with the government, had earlier harshly criticised Sudan for driving the homeless from the camps.
The involuntary movement appeared to have stopped after the homeless threw stones at Sudanese forces in protest, Mr Pronk said, expressing concern that the confrontation "might explode" and cause an "irreversible loss of life."
Mr Pronk also sharply criticized the rebels, saying they were looting aid convoys, closing roads and laying land mines.
The United Nations has negotiated for months with Khartoum on allowing aid and African Union monitors into Darfur. If Sudan continues to violate the agreements, some Security Council members are expected to renew a push for sanctions.
More than 1.5 million people have been driven from their homes by violence in Darfur in what the United Nations calls the world's worst ongoing humanitarian crisis.
The conflict erupted in early 2003 with a revolt by two rebel groups following years of skirmishes between African farmers and Arab nomads over land and scarce resources.
Militia then attacked villagers, with support from the Sudanese military, according to international observers. Khartoum denies this.