By Evelyn Leopold

UNITED NATIONS, Aug 14 (Reuters) - U.N. Secretary-General Ban Ki-moon said on Tuesday he plans to visit Sudan soon to expedite speedy deployment of a United Nations-African Union force for Darfur.

The secretary-general did not release a date but diplomats said the trip was expected in September, before Ban needed to be back in New York to prepare for the opening of the U.N. General Assembly session late that month.

Ban spoke to reporters after his monthly lunch with 15 U.N. Security Council ambassadors. The envoys told Reuters the secretary-general also would visit Chad and Libya in his first trip to the region since taking office in January.

Asked about the Darfur force, Ban said it was the top priority issue for him and the United Nations, adding: "I will try to expedite the speedy deployment of hybrid operation forces."

He also said he intended to follow up on peace negotiations being set up between the government and rebel leaders in Darfur, which he called "encouraging."

"I am also going to step up this political dialogue with all these regional groups as well as rebel group leaders," Ban said, without elaborating.

The U.N. Security Council last month authorized up to 19,555 military personnel and 6,432 civilian police for Darfur, which, if deployed, would be the world's largest peacekeeping force.

The immediate problem, U.N. officials say, is finding land and materials to build housing for advance troops that were to support 7,000 soldiers currently deployed by the African Union.

The agreement came after Ban held lengthy negotiations with Sudan's president, Omar Hassan Bashir, in part over the composition of any force sent into its western region to try to end four years of conflict that has killed an estimated 200,000 people and displaced 2.5 million.

AU officials said enough African nations had pledged infantry troops to give the hybrid mission an African character as Sudan had demanded. But their dates of arrival are not yet confirmed and some infantry contingents need to include mechanized battalions.

But U.N. spokeswoman Michele Montas said, "We still need specialized units, particularly in terms of technology, communications, transportation and these can be provided by other countries ... and they are not all of them African."

No Western country to date has promised military personnel, peacekeeping officials said. China and Pakistan are considering contributing engineering units.

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