(Adds likely Obama statement, quote from GlobalPost, background)
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.
U.S. authorities had previously said they were trying to verify the contents of the video, which was released on Tuesday.
Citing two unnamed U.S. officials, the Associated Press reported late on Tuesday that the video's authenticity had been confirmed.
The video, titled "A Message To America," also showed images of another U.S. journalist, Steven Sotloff, whose life Islamic State said depended on how the United States acts in Iraq.
Foley, 40, was kidnapped in Syria in 2012 while on his way to the Turkish border, according to GlobalPost. He had reported in the Middle East for five years and had been kidnapped and released in Libya.
Sotloff, who has written for TIME among other news organizations, went missing in northern Syria in 2013.
Foley's mother, Diane Foley, said in a statement late on Tuesday that her son gave his life to expose the suffering of the Syrian people.
The beheading came as a surprise and signaled a new front against the United States. Islamic State fighters in interviews with Reuters have hinted at more attacks to come. (Additional reporting by Mark Hosenball and Steve Holland; Writing by Jim Loney and Susan Heavey)