U.S. President Barack Obama has authorized reconnaissance flights over Syria as ISIS militants continue to gain ground in the war-ravaged nation, a U.S. official told CNN.

The flights - seen as a forerunner of possible U.S. airstrikes against ISIS in Syria - could begin at any point, the official said Monday.

"In general, when you're thinking about conducting operations like that, you certainly want to get as much of a view on the ground as you can," Pentagon spokesman Rear Adm. John Kirby said earlier Monday in a conversation with CNN's Wolf Blitzer.

"Satellites can provide you good visibility, but you always want closer eyes on target if you can," Kirby said, before Obama's decision was made public.

Concerns have been intensifying over the threat posed by ISIS after the extremist group posted a video last week showing the beheading of captive American journalist James Foley.

The United States has already been gathering intelligence on the locations of ISIS leadership and troops in Syria, two U.S. officials told CNN on Friday.

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