The U.S. is running out of bombs to drop on ISIS

The U.S. Air Force has fired off more than 20,000 missiles and bombs since the U.S. bombing campaign against ISIS began 15 months ago, according to the Air Force
, leading to depleted munitions stockpiles and calls to ramp up funding and weapons production.

As the U.S. ramps up its campaign against the Islamist terror group in Iraq and Syria, the Air Force is now "expending munitions faster than we can replenish them," Air Force chief of staff Gen. Mark Welsh said in a statement.

"B-1s have dropped bombs in record numbers. F-15Es are in the fight because they are able to employ a wide range of weapons and do so with great flexibility. We need the funding in place to ensure we're prepared for the long fight," Welsh said in the statement. "This is a critical need."

The bombing campaign has left the U.S. Air Force with what an Air Force official described as munitions depot stocks "below our desired objective."