More than three-quarters of European countries use American cloud computing services for vital national security functions, a report published Friday found, warning that the dependency could make governments vulnerable.

The analysis arrives as European governments increasingly fear that vital digital services could be subject to an American "kill switch" if tensions with U.S. President Donald Trump's White House escalate.

"Russia and Putin are waging a war against a European country in Ukraine... but we also had a US President threatening Denmark and Greenland," said Tobias Bacherle of the Brussels-based Future of Technology Institute (FOTI).

"This geopolitical shift needs to be taken in account," Bacherle said.

National security systems in 23 of the 28 countries studied "seem to rely on US tech", researchers found after looking at EU member states and Britain.

The researchers analysed public information from defence ministry websites, national media and EU and UK public procurement records to identify major cloud computing contracts with American providers including Microsoft, Google, Amazon and Oracle.

Of the countries studied, 16 of 28 "are at high risk to a potential US 'kill switch'", FOTI said, among them Germany, Poland and Britain (among Europe's key military powers).

Just one (neutral Austria, which is not a NATO member) was classified as lower risk.

Some capitals have already sought domestic or European options for tech procurement, prompting American firms to offer "sovereign" cloud computing services that they say are beyond Washington's reach.

But "This branding label fails to address the underlying dependencies", FOTI said, warning that such systems could still fall under the CLOUD act that allows American authorities to demand that companies hand over data stored abroad.

American companies could also be forced to cut off maintenance and security updates by U.S. sanctions, the researchers added.

Last year, Washington withheld from Ukraine services including satellite images from the American company Maxar after a public Oval Office sparring match between U.S. President Trump and his Ukrainian counterpart Volodymyr Zelensky.

"If you can do this to Kyiv, you can do it to London, Brussels, Paris and Berlin," said Katja Bego of the British think-tank Chatham House.

"It is a profound wake-up call moment. This is an existential national security risk."

FOTI pointed to early European steps to reduce dependencies, such as the move to open-source software by Italy's defence ministry or the north German state Schleswig-Holstein.

Meanwhile the Netherlands is working with telecoms firm KPN and France's Thales to build its own defence cloud.

"Even small changes or movement are a step in the right direction," FOTI's chief Cori Crider said. "You get resilience out of having at least some depth in an alternative."