Germany has asked the State Department to clarify US ambassador Richard Grenell's comment in an interview that he was seeking to "empower" European conservatives opposed to the political status quo.
At least one member of Angela Merkel's coalition government has accused the envoy of interfering in Germany's internal affairs, and figures on the left have called for him to be sent back to Washington.
But State Department spokeswoman Heather Nauert stood by the ambassador, insisting the United States has good relations with Germany and that Grenell had a right to make his views known.
"Ambassadors have a right to express their opinion," Nauert told reporters who asked whether Grenell had been expressing the views of President Donald Trump's administration.
"They're representatives of the White House, whether it's this administration or other administrations, and we hear them voicing their opinions," she said.
"And they're sometimes opinions that people may or may not like. And there is the right to free speech as well."
In an interview with news website Breitbart over the weekend, Grenell said he had seen a wave of support in Europe for conservative causes "because of the failed policies of the left."
"I absolutely want to empower other conservatives throughout Europe, other leaders," he said. "There's no question about that and it's an exciting time for me."
Grenell does not criticize Merkel's government, but dubs himself a "big fan" of neighboring Austria's "rock star" chancellor Sebastian Kurz, leader of a coalition of conservative and right-wing parties.
In a tweet sent from his personal account after controversy erupted, Grenell denied that he had been as US ambassador endorsing any European candidates or parties.
But he added: "I stand by my comments that we are experiencing an awakening from the silent majority — those who reject the elites and their bubble. Led by Trump."
On Wednesday, Grenell is due to hold a pre-arranged meeting with senior German official Andreas Michaelis.