Donald Trump could deactivate supplies of American F-35 fighter jets making their way to Germany at the push of a button known as a "kill switch", officials fear.
Germany is set to receive 35 of the world's most advanced fighter jets for €8.3 billion (£6.9 billion) next year in a deal with the US.
The ability of the US to flip a switch that would render them inoperable has long been the subject of speculation, but until now, it has not been proven.
But concern is mounting that Mr Trump might employ the same drastic action to enforce his political objectives on Ukraine.
This week, he froze military aid to Ukraineand paused intelligence-sharing to put pressure on Volodymyr Zelensky, the Ukrainian president, to convene peace talks with Russia and to sign a minerals deal with Washington.
On Saturday, reports surfaced that US-supplied F-16s had stopped working in Ukraine.
Though this was because of the suspension of critical support for radar systems rather than them being "switched off", Joachim Schranzhofer, the head of communications at Hensoldt, the German arms company, told Germany's Bild newspaper the "kill switch in the F-35 is more than just a rumour".
On Friday, Switzerland's department of defence denied reports suggesting the US could jam the F-35, and insisted that operators of the jet can use it "autonomously and independently at any time".
Gen Frederik Vansina, Belgium's chief of defence, said last week that the F-35 "is not a remote-controlled aircraft".