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

The Wall Street Journal 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 defence under the NATO umbrella, and appeared to catch Berlin off guard.

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

"We appreciate the cooperation with the US armed forces that has grown over decades. It is in the interest of both of our countries."

But he admitted ties had become strained under Trump.

"We are close partners in the transatlantic alliance. But it's complicated," Maas told Bild.

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 future.

But it comes amid tensions between the Trump administration and European allies over longstanding cooperation agreements.

Trump has been particularly critical of Germany in recent years, accusing the fellow NATO member of not spending enough on defence.

Germany hosts more US troops than any other country in Europe, a legacy of the Allied occupation after World War II.

Johann Wadephul, a senior member in Merkel's conservative CDU party, said the plan showed that the Trump administration was "neglecting an elementary leadership task: involving alliance partners in the decision-making process".

He warned that only China and Russia stood to gain from "discord" between NATO allies. "Washington should pay more attention to this," Wadephul said in a statement on Saturday.

- 'Security realignment' -

Peter Beyer, Chancellor Angela Merkel's coordinator for transatlantic relations, warned that Trump's plan to pull out some 9,500 US troops and their families would weaken "transatlantic bridges".

"The German-US relationship could be severely affected by such a decision from the US president," he told DPA news agency.

Rolf Muetzenich, leader of the parliamentary group of the centre-left SPD, Merkel's junior coalition partner, told the Funke newspaper group that the US troop reduction could lead to "a lasting realignment of security policy in Europe".

Polish Prime Minister Mateusz Morawiecki said Saturday that he hoped some of the troops moved out of Germany could be reassigned to Poland.