US forces move into Syria: Obama paves way for airstrikes on ISIS after sending in spy planes - despite Assad warning that any attack will be seen as 'aggression'

Barack Obama has paved the way for airstrikes against ISIS targets in Syria after spy planes started operating in the country on his command, according to U.S officials.  

While the White House says military action inside Syria has not yet been approved, additional intelligence on the militants is considered vital before any move can be made - especially in the northern city of Raqqa, which ISIS has named the capital of its self-declared caliphate. 

The move comes despite warnings from Syria's embattled president Bashar Assad that any airstrike in the country not given his express permission would be considered an act of aggression.

Obama has so far sought a limited military campaign in neighbouring Iraq focused on protecting American diplomats and Iraqi civilians under direct threat from the brutal Islamist militant group.

But officials have not ruled out escalating military action against ISIS, which has increased its overt threats against the United States in recent weeks.