The United States Air Force, which has launched over 20,000 missiles and bombs since the start of the bombing missions against ISIS over a year ago, revealed that its munitions stockpiles are now depleted.

The U.S. Air Force is now calling for a ramp-up for the funding and production of weapons, as it is using up munitions faster that they are being replenished.

"This is a critical need," said Gen. Mark Welsh, the chief of staff of the U.S. Air Force, as B-1 bomber aircraft drop a record number of bombs and F-15E fighter jets use up a variety of weapons.

"We need the funding in place to ensure we're prepared for the long fight," Welsh said.

An official said to CNN that the bombing missions against ISIS have left the U.S. Air Force with stocks of munitions below the desired levels, and that the U.S. Air Force has sent in a request for additional funds to secure more Hellfire missiles while developing strategies to increase munitions production and replenish stocks.

However, the official said that between the initial expenses until the actual replenishment of the munitions stock of the U.S. Air Force, it could take up to four years.

American military aircraft have launched over 75 percent of the total 8,556 airstrikes against ISIS in Syria and Iraq. In November alone, 65 percent of the coalition airstrike attacks in Syria and Iraq saw one or more aircraft use munitions, which is a significant improvement from the summer when half of the aircraft were returning from the missions still with complete munitions onboard.

U.S. pilots have used their weapons in less than 50 percent of the almost 18,000 sorties carried out from the beginning of the year until the end of October, which is already up compared to the figure of the pilots firing their weapons in only about 33 percent of the sorties last year.

The number of missiles and bombs used by U.S. Air Force comes amid criticism against the Obama administration that it has been timid in the war against ISIS.