U.N. Secretary General Ban Ki-moon called on African leaders to help end worsening violence in Darfur by backing the urgent deployment of a joint U.N. and African peacekeeping force.

In a keynote speech at the opening of an African Union summit in Ethiopia Monday, he said leaders should work together to end the deadlock created by Sudan's refusal to allow U.N. peacekeepers into the violence-wracked western Sudan region. Darfur's violence has spilled into neighboring Chad and Central African Republic.

Ban also called for aid workers to be allowed to operate in Darfur, as humanitarian agencies said their operations are on the brink of collapse.

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A leading French aid group said Monday it was pulling out of the western region of Sudan because of insecurity. Six other international charities said in a statement Sunday their work will soon be paralyzed unless urgent action is taken to end the rising violence.

"We must work to end the violence and scorched earth policies adopted by various parties, including militias, as well as the bombings which are still a terrifying feature of life in Darfur," Ban told African leaders who included Sudanese President Omar al-Bashir. "The toll of the crisis remains unacceptable."

More than 200,000 people have been killed and 2.5 million displaced in Darfur since rebels took up arms against the central government in 2003. Khartoum is accused of retaliating indiscriminately against civilians and supporting janjaweed paramilitary groups blamed for some of the worst atrocities in the conflict.

The Sudanese government signed a peace agreement with one Darfur rebel faction in May, but violence has worsened in the region. Sudan and Chad also have been trading accusations of supporting each others' rebel groups.

Khartoum opposes a U.N. Security Council resolution that calls for some 22,000 U.N. peacekeepers to replace or absorb an African force. The AU has 7,000 peacekeepers struggling to end the fighting.

The U.N. chief, on his first visit to Africa since taking over from Kofi Annan on Jan. 1, is expected to hold talks later Monday with al-Bashir.

Sudan has said it intended to try to secure the rotating chairmanship of the African Union, though international organizations have said Sudan does not deserve the post and African diplomats have indicated it will go to another country.

Chad President Idriss Deby said it was unlikely that Sudan would receive the chairmanship, but did not say if his country would oppose the move. Darfur rebel leaders have said they would stop considering the current AU peacekeeping mission as an honest broker in Darfur if al-Bashir was selected.

In a statement released Monday in his homeland of South Africa, Nobel peace laureate Desmond Tutu also spoke out against the possibility of Sudan taking the A.U. chairmanship.

"The A.U. cannot allow itself to comfort the oppressor," said Tutu, the retired Anglican archbishop of Cape Town who was awarded the Nobel peace prize in 1984 for his efforts to end apartheid in South Africa. "I appeal to those leaders meeting at the A.U. summit to stand up to tyranny and stand by the people of Darfur."

Alpha Oumar Konare, the A.U.'s chief executive, called on the Sudanese government to stop aerial bombing in Darfur.

"Peace in Sudan means peace in Chad," Konare told summit delegates who included African leaders, foreign ministers and diplomats in the Ethiopian capital Addis Ababa.

The summit will also focus on putting together an African peacekeeping force for Somalia. Konare said that chaos would return to Somalia, a country that has been without a government since 1991, without a rapid deployment of peacekeepers to the troubled nation.

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