US aircraft carrier Theodore Roosevelt Photo: Xinhua

Germany on Sunday voiced concern at reports that US President Donald Trump plans to cut the number of US troops stationed in Germany, amid fears it could weaken a key pillar of NATO's defense in the region.

German Foreign Minister Heiko Maas said both countries stood to gain from close cooperation even if the transatlantic relationship had become "complicated" under Trump. 

Other senior politicians in Berlin were blunter, slamming the plan as the latest blow to US-German ties and a potential security risk.

"Should it come to the withdrawal of part of the US troops, we take note of this," Maas told the Bild am Sonntag daily. 

Peter Beyer, Chancellor Angela Merkel's coordinator for transatlantic relations, warned that "the German-US relationship could be severely affected" by Trump's decision.

The Wall Street Journal and other media reported on Friday that Trump had ordered the Pentagon to slash the number of US military personnel by 9,500 from the current 34,500 permanently assigned in Germany. 

Such a move would significantly reduce the US commitment to European defense under the NATO umbrella, and appeared to catch Berlin off guard.

But Maas admitted ties with the Trump administration had become strained.

There was no immediate confirmation from US officials about the alleged plan to slash US troop numbers in Germany and cap them at 25,000 in the future.

But Trump's lukewarm support of long-standing cooperation agreements with European allies has long caused alarm on the continent.

AFP