WASHINGTON, Aug 20 (Reuters) - The FBI believes the Islamic State video purporting to show the beheading of American journalist James Foley is authentic, GlobalPost reported on Wednesday.

"The FBI on Wednesday morning told the Foley family they believe the video is authentic," according to GlobalPost, a Boston-based online publication that employed Foley as a freelancer.

U.S. President Barack Obama, who is on vacation on the Massachusetts island of Martha's Vineyard, is likely to comment on the video later in the day, a U.S. official told Reuters.

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