The gains made in recent months by the group calling itself the Islamic State have bolstered its long-term ambitions to attack the West, including the U.S., and the group has become such a potent force that traditional counterterrorism tactics aren't sufficient to eliminate it, U.S. intelligence officials said Thursday.

Officials painted a bleak assessment of the threat posed by a group that was largely counted out just four years ago, when it was known as al Qaeda in Iraq.

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