Official: ISIS is "credible alternative to al Qaeda"

(CNN) - The Islamic State terror group is now "a credible alternative to al Qaeda" that is "expanding its presence" with foreign fighters returning from Syria, and possibly Iraq, to their home countries, a U.S. intelligence official said Thursday.

The official, who declined to be identified due to the sensitive nature of the information, has direct knowledge of the latest intelligence on the group, also known as ISIS or ISIL.

According to an assessment, the group has grown in size since the spring and its takeover of Mosul in northern Iraq as more fighters from around the world have mainly traveled to Syria to join its ranks.

The United States believes that while the group remains largely focused on its brutal takeover of large areas of Iraq, there is also an "expansion of its external terrorist ambitions."

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