Since the war began just over five months ago, the US military has used up nearly 80 percent of its so-called THAAD missiles, which are essential for stopping ballistic missile attacks, according to CNN sources with insight into the stockpiles.
In addition, the United States is estimated to have used up about two-thirds of its Patriot air defence missiles during the same period, according to a report from the think tank Center for Strategic & International Studies, CSIS.
The US has used up three years of production of the missiles in a short time, says Johan Granholm, a teacher at the Swedish National Defence University.
During the war, these have been used to defend US bases and ships in the Middle East, as well as US allies in the region, against Iranian attacks.
Greater risks
It's not just defensive missiles that are running low. Two key offensive missiles are also running out as the war drags on. The US has essentially used up all of its ATACMS and PrSM systems, Reuters reported on Tuesday.
If the US does not want to end the war, which it does not seem to want to, it will be much less intense, says Johan Granholm.
The alternative is to use manned fighter aircraft much closer to the front and at shorter ranges, and then you expose a large part of your air force, both people and machines, to great risks.
Concern among allies
Several of America's allies around the Persian Gulf depend on American air defence systems. They have expressed concern that the shrinking stockpiles risk weakening their ability to withstand Iranian retaliatory attacks if President Donald Trump decides to escalate the war, according to CNN.
The lack of air defence missiles has been pointed out as one of the reasons why Trump chose not to launch massive bombings of Iran last weekend.
American arms companies have been urged to increase production of missiles, but it is a slow process.
It takes two to five years to build an advanced missile, says Johan Granholm.
The manufacturers have a couple of hundred subcontractors who make very specialised parts that will be included in the missile. This means that you have to increase the production rate at several hundred different factories, which is difficult.
