U.S. Senator Bernie Sanders has slammed President Donald Trump for his disparaging remarks about Germany, saying that publicly attacking Chancellor Angela Merkel was “unacceptable.”

“To insult Germany, a long-standing ally, is something that many of us feel very uncomfortable with,” Sanders said in a dpa interview conducted ahead of his German book launch in Berlin on Wednesday.

He said: “when there are differences, they should be solved quietly.

“It is not acceptable to my mind that the president publicly attacks the chancellor.”

U.S.-German relations have come under strain by Trump’s repeated criticism of the country’s trade deficit and his threat to impose a hefty tax on German carmakers for selling vehicles in the U.S. market that were produced elsewhere.

Trump also has demanded that Germany meet the NATO military alliance’s defence spending target of two per cent of gross domestic product and criticised Merkel’s open-door migration policy.

Sanders, who lost the Democratic Party nomination in the 2016 presidential elections to Hillary Clinton, is in Berlin to present his book “Our Revolution: A Future to Believe in.”