Berlin (dpa) - The US ambassador to Germany, Richard Grenell, has lashed out again at low levels of German defence spending, calling it "offensive" and repeating a threat to withdraw US troops from the country.

"It is actually offensive to assume that the US taxpayer must continue to pay to have 50,000-plus Americans in Germany, but the Germans get to spend their surplus on domestic programmes,” Grenell said in an interview with dpa on Friday.

During a visit by Polish President Andrzej Duda to Washington in June, Trump suggested redeploying US troops from Germany to Poland.

The US ambassador to Poland, Georgette Mosbacher, tweeted Thursday: "Poland meets its 2 per cent of GDP spending obligation towards NATO. Germany does not. We would welcome American troops in Germany to come to Poland."

Grenell's latest remarks came ahead of Trump's planned European trip.

“President (Donald) Trump is right and Georgette is right ... Multiple presidents have asked the largest economy in Europe to pay for their own defence. This is a multi-year, multi-administration ask," Grenell said.

"I think that we are at the point where Americans and the US president are reacting at the multi-administration blow-off," the outspoken ambassador, who is a close associate of Trump's, said.

Germany is host to the largest number of US troops in Europe and has the largest contingent of US troops abroad after Japan.

US command centres for its troops in Europe and Africa are in Stuttgart and the most important US air base in Europe is at Ramstein in the south-west of the country.

The southern state of Bavaria is home to one of the largest US military exercise grounds in Grafenwoehr.