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KABUL, Aug 26 (Reuters) - A U.S. military helicopter carrying ballot boxes packed with votes from last week's Afghan election accidentally dumped some of them somewhere over the rugged mountains of Nuristan, officials said on Wednesday.

Some 50 ballot boxes were being carried in a "sling load" beneath a Chinook helicopter from the provincial capital Paroon when the accident happened, they said.

"During transport, apparently some of the materials came out of the load as it was travelling," U.S. army Lieutenant Tommy Groves told Reuters.

He said 15 of the 25 boxes that fell were recovered. The Independent Election Commission said the votes had already been counted and the totals added to the national tally.

A partial tally based on 10 percent of the votes counted has President Hamid Karzai and rival Abdullah Abdullah running neck-and-neck, suggesting a close race headed for second round.

(For more Reuters coverage of Afghanistan and Pakistan, see: http://www.reuters.com/news/globalcoverage/afghanistanpakistan) (Reporting by Hamid Shalizi; Editing by Nick Macfie)

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