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By Evelyn Leopold

UNITED NATIONS, July 31 (Reuters) - The U.N. Security Council voted unanimously on Tuesday to authorize up to 26,000 troops and police in an effort to stop attacks on millions of displaced civilians in Sudan's Darfur region.

Expected to cost more than $2 billion in the first year, the combined United Nations-African Union operation aims to quell violence in Darfur, where more than 2.1 million people have been driven into camps and an estimated 200,000 have died over the last four years.

The resolution allows the use of force in self-defense, to ensure freedom of movement for humanitarian workers and to protect civilians under attack.

But the measure, which has been watered down several times, no longer allows the new force to seize and dispose of illegal arms. Now they can only monitor such weapons.

Gone also is a threat of future sanctions but British Prime Minister Gordon Brown warned on Tuesday that "if any party blocks progress and the killings continue, I and others will redouble our efforts to impose further sanctions."

"The plan for Darfur from now on is to achieve a cease-fire, including an end to aerial bombings of civilians; drive forward peace talks ... and, as peace is established, offer to begin to invest in recovery and reconstruction," he said on a visit to the United Nations.

Specifically, the text authorizes up to 19,555 military personnel and 6,432 civilian police.

The resolution calls on member states to finalize their contributions to the new force, called UNAMID or the United Nations-African Union Mission in Darfur, within 30 days. UNAMID would incorporate the under-equipped and under-financed 7,000 African Union troops now in Darfur.

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

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