AMSTERDAM, Aug 4 (Reuters) - Former United Nations envoy for Sudan Jan Pronk is concerned about the effectiveness of a U.N. force for Darfur as the government in Khartoum has obstructed previous U.N. resolutions, Dutch media reported on Saturday.

The U.N. Security Council authorized on Tuesday up to 26,000 troops and police for Darfur and approved the use of force to protect civilians in Sudan's arid western region, where an estimated 200,000 have died over the past four years.

"It will take a very long time before that mission is fully operational," said Pronk in an interview with Dutch daily Trouw.

Pronk, who headed the U.N.'s Sudan mission for nearly two years before he was expelled from the country last October for criticising its government, said the Sudanese cabinet had a history of obstructing U.N. missions.

"(The government) has not executed any of the U.N. Security Council's resolutions. Everything, they have flouted everything," the paper quoted Pronk as saying.

Sudan promised on Wednesday to cooperate with the U.N. force, UNAMID, which will incorporate 7,000 African Union troops now in Darfur, after it took part in talks on drafting the new U.N. resolution.

The planned U.N. force would have difficulty finding troops as African nations would not be able to provide the necessary numbers, Pronk was quoted as saying in Dutch daily NRC Handelsblad.

He expressed concerns that the U.N. force would not be allowed to seize and dispose of illegal arms, saying the U.N. resolution had been watered down "enormously".

"Everything has been taken out. For refugees to return home, you first have to drive out the militants. That is a job for soldiers. This is also not addressed," Pronk told Trouw.

Rape, looting, murder and government bombardments drove millions from their homes in Darfur, where mostly non-Arab rebels took up arms in early 2003, accusing Khartoum of neglecting their desert region. The rebels have now split into a dozen groups, many fighting one another.

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