She warned in mid-June that Germany is “in the crosshairs of jihadist organisations, in particular Isis and Isis-K”. 

Isis-K is the Afghanistan-based wing of the Islamic State terrorist group. It claimed responsibility for the massacre at Moscow’s Crocus City Hall in March and is suspected of planning terrorist attacks in Cologne over Christmas.

The head of domestic intelligence also warned that the group could send operatives to Europe disguised as Ukrainian refugees.

France also raised its threat level warning this month ahead of a possible attack on the Paris Olympics.

The United States European Command refused to comment on specific threats for security reasons, but in a statement on Sunday it said it was “constantly assessing a variety of factors that play into the safety of the US military community abroad. As part of that effort, we often take additional steps to ensure the safety of our service members”.

About 52 Fighter Wing airmen at the Spangdahlem air base, also in western Germany, were prohibited from wearing their uniforms off base and told to commute in civilian clothing, the Stars and Stripes military news site reported.