By Louis Charbonneau

UNITED NATIONS (Reuters) - The head of the joint U.N.-African Union peacekeeping mission in Sudan's conflict-torn Darfur region, who some diplomats say has been ineffective, is stepping down, the U.N. said on Tuesday.

The world body is working closely with the AU to find a replacement for Rodolphe Adada, a former foreign minister of the Republic of Congo, U.N. spokeswoman Marie Okabe said.

Other U.N. officials told Reuters on condition of anonymity that the United Nations and AU were considering several candidates from different parts of Africa.

The U.N.-AU peacekeeping force in Darfur, known as UNAMID, said in a statement that Adada's resignation takes effect on August 31. Diplomats and U.N. officials said he was expected to return to politics in his home country.

General Henry Anyidoho of Ghana, deputy head of UNAMID, will head the mission until a permanent replacement for Adada is named, U.N. officials said.

The conflict in Sudan's western Darfur region has been going on for more than six years. The United Nations says as many as 300,000 people have died since 2003, compared to Khartoum's official death toll of 10,000. The world body also says some 4.7 million people in Darfur rely on aid to survive.

UNAMID was established by a U.N. Security Council resolution passed in July 2007. Adada has led the mission from the start, overseeing its slow and difficult deployment.

At the end of June, just over 60 percent of UNAMID's planned full strength of 26,000 troops and police had been deployed in Darfur, an area roughly the size of France. The U.N. hopes 90 percent will be on the ground by the end of the year.

SLOW DEPLOYMENT

The slow deployment of UNAMID has frustrated the United Nations, member states, aid workers and activists lobbying governments to do more to help the people of Darfur. Diplomats and activists have also complained that the United Nations has done too little to revive the stalled Darfur peace process.

John Prendergast, a former U.S. State Department official and co-founder of the Enough Project, an anti-genocide group, said UNAMID has been widely perceived as a failure.

"There is an urgent need to construct a more credible and effective peace process backed by stronger leverage," he said. "U.N. efforts have meandered without laying down a serious peace proposal. In the absence of diplomatic leadership, it is not surprising that most analysts view UNAMID as ineffective."

Western diplomats said Adada's job was a difficult one, since Sudanese President Omar Hassan al-Bashir, an indicted war crimes suspect over Darfur, created bureaucratic obstacles that have slowed UNAMID's deployment.

The diplomats said there was a broad consensus that Adada did not try hard enough to speed up UNAMID deployment. The U.N. secretariat, they said, was also dissatisfied with his performance, but since the appointment was made jointly by both the U.N. and the AU it was not possible to get rid of him.

"He hasn't been the most effective head of a mission," a Western diplomat said on condition of anonymity. "We were unhappy with him for a long time but the AU protected him."

A U.N. official said it was disingenuous for diplomats from Western countries that have refused to provide military hardware desperately needed by UNAMID -- above all helicopters -- to criticize Adada for not doing a better job.

U.N. Secretary-General Ban Ki-moon praised Adada.

In his reply to Adada's letter of resignation, quoted in the online version of Sudan Tribune newspaper, Ban said he "led UNAMID with distinction during its most challenging initial deployment phase and in an environment of unprecedented difficulty."

(Editing by Mohammad Zargham)