The most worrying shortage concerns Patriot missiles, which have the ability to shoot down, for example, Russian ballistic missiles, a source within US forces in Europe told the AP.
The news agency's second source, within NATO, confirms that stocks in Europe have shrunk to low levels.
There are far too few missiles to stop a more extensive ballistic attack, and even a single attack would cause difficulties, the American source said.
Downplays attack risk
CIA Director John Ratcliffe visited Russia earlier this week under high-profile circumstances. According to sources cited by The Wall Street Journal, the purpose was to warn Russia against attacking NATO countries.
However, European intelligence services see no signs that Russia is planning any military attack on the continent's NATO countries, a senior EU source told AFP on Thursday.
That picture is also presented by Donald Trump, who claims to have had "good conversations" with Russia's Vladimir Putin.
He will not attack NATO territory, the American president said on Thursday.
Used in the Middle East
In connection with the Iran war, the US has moved many of its air defence missiles to the Middle East to shoot down drones and prevent missile attacks on American bases, among other things. Stocks have also been somewhat depleted when equipment was donated to Ukraine.
President Donald Trump - who has denied that the US has any shortage of missiles - has ordered American arms companies to increase production. He has also promised Ukrainian President Volodymyr Zelenskyy that the country will be allowed to produce its own Patriot missiles.
