US F15-E jets in northern Iraq (file)
US F15-E jets in northern Iraq (file) Reuters

Airstrikes against Islamic State in Iraq and Syria (ISIS) have totaled $780 million dollars, a think tank influential with the Pentagon reports Monday - and that is the low end of the estimate. 

According to the Center on Strategic and Budgetary Assessments (CSBA), the US's resurged "war on terror" has already cost between between $780-$930 million dollars - and that only in the six-week period between the beginning of airstrikes and last Wednesday.

The estimate far outweighs US Defense Secretary Chuck Hagel's estimate Friday that the war is costing some 10 million dollars per day - which would total to $420 million - and calls into question the rising costs of an all-out war in the event Congress declares the airstrikes as such in November. 

The cost, as quoted by Iraq war architect Frederick Kagan, could top $13 billion - 19 billion per year, according to the British Guardian. An "in between" compromise - a combination of airstrikes and limited ground troop deployment - could have lower, but still heavy, costs - at between four and seven billion per year. 

According to the Pentagon, US aircraft have carried out 178 air strikes against IS targets in Iraq since August 8.

A separate offensive was launched against ISIS in Syria earlier this month, although it has been deemed "ineffective" by local Kurdish Pashmerga fighters on the ground.