UN Secretary General Ban Ki-moon expressed disappointment Thursday that Sudan's president reneged on a promise to allow a United Nations human rights team to visit Darfur to assess alleged atrocities.

He said the deteriorating situation in the vast conflict-wracked western region of Sudan is unacceptable.

Ban said that during a meeting with Sudanese President Omar al-Bashir at last month's African Union summit in Addis Ababa, Ethiopia, "he said he would issue visas to the fact-finding mission — he said he would have no problem."

"If he believes that there is no problem, then he should be able to receive the human rights fact-finding mission," the secretary-general said.

The 14-member mission has been stuck in Addis Ababa because Khartoum has failed to give them visas.

Mission leader and Nobel peace laureate Jody Williams said Wednesday the team couldn't wait any longer and would carry out its assessment from outside the country and submit a report to the Human Rights Council in Geneva.

"I am very much disappointed by the decision of the Sudanese government," Ban said. "I urge again that the Sudanese government fully co-operates with the unanimous decision of the Human Rights Council" to send a fact-finding team to Darfur.

More than 200,000 people have been killed and 2.5 million been chased from their homes in Sudan's remote western region since 2003, when people from ethnic African tribes rose up against the central government.

Khartoum is accused of having responded with indiscriminate killings by unleashing the janjaweed militias of Arab nomads — blamed for the worst atrocities in Darfur — in a conflict that the White House and others have labelled genocide. The Sudanese government denies these charges.

Sudanese officials agreed in November on a three-phase UN package to help end the escalating violence that culminates with the deployment of a 22,000-strong African Union-UN "hybrid" force.

But al-Bashir said last month that UN troops were not required in Darfur because the 7,000-strong African Union force on the ground could maintain order.

Ban told reporters after his monthly luncheon with the Security Council, which focused on the Darfur conflict, that he was still waiting for an official reply from al-Bashir to his Jan. 24 letter outlining the UN's detailed positions on the force, command and control, and funding.

"With an affirmative answer, we can pave the way immediately to the introduction of an AU-UN hybrid mission," Ban said.

"This continuing deteriorating situation in Darfur is just unacceptable," he said.

The secretary-general said he is waiting for a report from his special envoy, Jan Eliasson, who is now in Darfur with AU envoy Salim Salim.

"On the basis of the report … I will take a future course of action on this matter," Ban said.