The report cites Associated Press sourcing.
The shortage has raised concerns about NATO countries' vulnerability to a potential Russian attack.
A U.S. defense official said the most concerning shortfall involves Patriot missile interceptors.
Patriot interceptors can shoot down Russia's high-speed ballistic missiles.
A NATO official confirmed low Patriot inventories among American and NATO forces in Europe.
The Iran war reaches the six-month mark on Friday, August 28.
A U.S. defense official said NATO could be forced to take punch after punch in the mouth in a Russian ballistic missile assault.
A U.S. official said American forces in Europe definitely do not have enough Patriots to stop a sustained ballistic missile attack.
The U.S. supplied hundreds of Patriot missiles to Ukraine over the 4 1/2-year conflict.
A U.S. official said Europe would have very limited capability against a single ballistic missile strike or a stray Russian missile.
Arnold said the Patriot supply to Ukraine was planned and reasonably measured.
Officials said the Europe interceptor shortage is largely driven by President Donald Trump's war with Iran.
Arnold said Middle East stocks have been unexpectedly and quickly drained, exposing supply-chain constraints.
U.S. missile stocks in Europe have been moved to the Middle East.
U.S. and Gulf allies have fired large numbers of interceptors, including Patriots, against Iranian drones and missiles.
Some of those Iranian attacks have killed and wounded American troops.
The Patriot shortage is most acutely felt in Ukraine.
U.S. forces in Europe also face critical shortages of ATACMS missiles.
CIA Director John Ratcliffe went to Moscow this week to warn Russia not to attack, the Wall Street Journal reported.
Officials said the overall munitions picture in Europe is less dire than air defenses alone.
Trump said he did not want to comment.
O'Donnell said NATO is able to block punches.
Experts say Europe has few alternatives for shooting down Russia's more advanced ballistic missiles while Patriot stocks are low.
Trump said Russia is not going to attack.
O'Donnell said NATO has enough air-defense munitions to defend itself and to donate to Ukraine.
Patriot stockpiles are projected to rebound by 2030.
Parnell said the U.S. has everything required to strike at the time and place of the president's choosing.
Parnell said the U.S. has executed successful operations across combatant commands while keeping a deep, ready arsenal.
Ukrainian officials are asking for more Patriots to defend against Russian missile attacks.
France and Italy's SAMP/T NG is cited as one alternative to Patriot.
France has promised to speed up delivery of SAMP/T NG to Ukraine.
A Russian ballistic missile attack against NATO countries is not considered imminent.
Previous U.S. Patriot transfers to Ukraine have drawn down some of the American supply.
The manufacturer says the upgraded SAMP/T is better able to counter ballistic missiles.
European and NATO officials say Russia could be in a position to attack alliance countries by 2030.
Kyiv says only U.S.-made Patriots can stop Russia's advanced ballistic missiles.
The new SAMP/T NG iteration has not yet been battle-tested.
Russia's war in Ukraine could hinder a missile attack on Europe because fighting an air war on two fronts would be a challenge.
Russia's advanced ballistic missiles are increasingly getting through Ukrainian defenses.
The current SAMP/T has a smaller range than the Patriot.
A Russian barrage overnight into Thursday got through Ukrainian defenses.
CSIS says the Iran conflict has used up about 65 percent of U.S. Patriots, roughly 1,500 of 2,330 interceptors.
About 600 Patriot missiles, including some destined for Europe, should be made this year.
Ed Arnold said the Iran war was the tipping point for shrinking Patriot inventories.
The U.S. production goal is 2,000 Patriot missiles a year by 2030.
NATO says Europe's first Patriot missile production facility is expected to open in Germany in September.
Deliveries from that factory may begin early next year.
First deliveries are to go to countries that already ordered, including Germany, the Netherlands, Romania, and Spain.
ATACMS is a medium-range ballistic missile designed to strike behind enemy lines.
Some European militaries have their own offensive missiles, including Taurus and Storm Shadow.
The Pentagon said it would not discuss weapons stockpile levels for security reasons.
Advanced ballistic missiles can be combined with swarms of inexpensive drones to overwhelm defenses.
Ukraine and some allies already use systems that can destroy slower-moving drones without expensive missiles.
Ballistic missiles dive steeply toward targets at more than five times the speed of sound.
One Patriot battery can cover only a limited area, such as a small military base but not a whole city.
Warships may be too far from a target to protect it and could risk getting sunk.
Ukraine has received Patriots from American and European stockpiles since Russia's 2022 invasion.
The U.S. provided about 600 Patriot interceptors during the Biden administration.
Arnold said Germany has bought expensive Patriot systems but lacks ammunition to fire from them.
Arnold said Moscow likely cannot carry out a sustained air assault on a NATO target while launching almost nightly attacks on Ukraine.
Ukraine's recent strikes into Russia have exposed weaknesses in Russia's air defenses.
Those exposed weaknesses could prompt Putin to think twice about provoking a retaliatory attack.
The U.S. defense official and NATO official spoke on condition of anonymity.
The Patriot system tracks a missile, predicts its path, and launches within minutes.
U.S. and European warships can serve as mobile air defense.
Germany's defense ministry said it does not comment on stockpiles.
None yet.