President Barack Obama and some administration officials have hailed recent military gains against the Islamic State group in Iraq and Syria, but other U.S. officials and outside experts warn that the U.S.-backed air and ground campaign is far from eradicating the radical Islamic group, and could even backfire.

While Islamic State's defeats in Iraq and Syria have erased its image of invincibility, they threaten to give it greater legitimacy in the eyes of disaffected Sunni Muslims because Shiite and Kurdish fighters are a major part of the campaign, some U.S. intelligence officials argue.

A second danger, some U.S. officials said, is that as the group loses ground in the Iraqi city of Fallujah and elsewhere, it will turn increasingly to less conventional military tactics and to directing and inspiring more attacks against "soft" targets in Europe, the United States and elsewhere.