WASHINGTON, Aug 1 (Reuters) - The U.S. Defense Department said on Wednesday it will send about 1,700 National Guard troops to Afghanistan to replace U.S. forces assigned to train the Afghan army and police.

The troops, from the 27th Infantry Brigade Combat Team of the New York Army National Guard, will begin arriving in Afghanistan in December, with the majority of the deployment due to occur in mid-2008 as part of a 12-month mobilization.

They will replace a unit from the South Carolina Army National Guard scheduled to return home.

U.S.-led troops invaded Afghanistan in 2001 and overthrew the country's Taliban rulers after they refused to hand over al Qaeda leader Osama bin Laden and other militants involved in the Sept. 11 attacks on New York and Washington.

Washington has spent billions of dollars to rebuild Afghanistan and create an Afghan security force capable of battling Taliban insurgents as the country suffers its worst violence since the 2001 invasion.

Afghanistan's army disintegrated in 1992 after Western-backed mujahideen fighters overthrew an earlier Soviet-backed government.

The Defense Department currently has about 2,300 troops involved in the training of Afghan soldiers and police, as part of an 11,000-troop deployment under Operation Enduring Freedom.

Another 14,000 U.S. troops are in the country under NATO command.

About 76,000 Afghan police officers and 39,000 army soldiers have completed training, a defense official said. Plans call for the training of another 6,000 police and 31,000 soldiers by the end of 2008.

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