The US has run out of bombs to bomb ISIS.

The U.S. Air Force's ammunition stockpiles are running low after using more than 20,000 missiles and bombs since beginning its airstrike campaign against the Islamic State (IS) group 15 months ago.

A US Air Force F-22 Raptor participated in its first strike mission against ISIS targets in Syria in 2014. Photo: CNN

CNN, citing a statement from US Air Force Chief of Staff General Mark Welsh, reported that as the US intensifies its campaign against ISIS in Iraq and Syria, the Air Force is expending more ammunition than it can replenish.

"The B-1s have dropped record numbers of bombs. The F-15Es have been deployed because they can use a wide variety of weapons with great flexibility. We need the budget to ensure readiness for this long-term conflict," Welsh said. "This is an urgent need."

He said the US Air Force has requested additional funding to supplement its Hellfire missiles and is implementing plans to increase weapons production. However, this process could take up to four years.

The public disclosure of the number of missiles and bombs the U.S. military used in airstrikes against ISIS militants comes as Republican lawmakers continue to criticize President Barack Obama's administration. They argue that he has been too timid in the fight against the terrorist group and call for loosening of rules to allow for deeper U.S. military involvement.

American pilots used weapons in less than half of the nearly 18,000 sorties in the first 10 months of this year. However, this number has increased compared to 2014, when pilots bombed ISIS targets in only one-third of their sorties.

According to VNE