Berlin (CNN) Politicians on both sides of the Atlantic have criticized US Ambassador to Germany Richard Grenell for giving an interview to far-right news platform Breitbart in which he said he wants to "empower" European conservatives.

Grenell, who took up his post last month and is a close ally of US President Donald Trump, also described Germany's NATO spending as "woeful" and criticized Chancellor Angela Merkel's immigration policy.

The ambassador has been rebuked for politicizing diplomacy at a time when US-German relations are already strained over Trump's withdrawals from the Iran deal and the Paris climate accord, along with his imposition of steel and aluminum tariffs on the European Union.

But Grenell has since insisted that his remarks have been misinterpreted. "The idea that I'd endorse candidates/parties is ridiculous," he wrote on Twitter early Monday.

Germany's foreign ministry has asked the US for clarification of Grenell's comments, according to spokesman Christofer Burger.

A State Department official, speaking on background, said, "Ambassador Grenell clarified his comments via Twitter and noted it is not US policy to endorse candidates or parties. He was making general observations in the interview."

Grenell: Austrian chancellor is 'rock star'

At a sit-down interview with Breitbart London in Berlin, Grenell said, "There are a lot of conservatives throughout Europe who have contacted me to say they are feeling there is a resurgence going on."

He added: "I absolutely want to empower other conservatives throughout Europe, other leaders. I think there is a groundswell of conservative policies that are taking hold because of the failed policies of the left."

Grenell went on to attribute this "resurgence" to the election of Trump and said that "support is massive" for conservative policies on migration as well as tax cuts and reducing bureaucracy.

He did not say whether he included Merkel -- of whom he has previously said he is a "big fan" -- as one of the conservatives he supported. But by describing Austrian Chancellor Sebastian Kurz -- a hard-right conservative who wants to clamp down on migration and support for asylum seekers -- as a "rock star" and describing himself as a "big fan," Grenell made his position clear.

In addition to criticizing Germany's NATO spending, Grenell alleged that Germany is not ready for military action -- both indirect criticisms of a leader who has been in power for 13 years -- while also attacking the Chancellor directly for the decision to allow migrants and refugees into the country in 2015.

"Donald Trump talks a lot about chain migration, and that is actually the issue here in Germany — it's chain migration," Grenell said. "Many migrants have been allowed to come in, that was the policy of Chancellor Merkel."

Sebastian Kurz became Chancellor of Austria in December.

'Grenell isn't behaving like a diplomat'

US Senator Chris Murphy said on Twitter that he had raised concerns with Grenell in the past about politicizing the ambassador post. "He personally assured me that once he became Ambassador he would stay out of politics," Murphy wrote. "This interview is awful - Ambassadors aren't supposed to 'empower' any political party overseas."

When I raised concerns to Grenell about politicizing this post, he personally assured me that once he became Ambassador he would stay out of politics. This interview is awful - Ambassadors aren't supposed to "empower" any political party overseas. https://t.co/i8oOhqEk5k — Chris Murphy (@ChrisMurphyCT) June 3, 2018

Metin Hakverdi, Social Democrat member of the German parliament, told The Guardian he agreed with Murphy and said he was "irritated" by Grenell's comments.

"In the past, Germany was fortunate to have had great US ambassadors who built bridges and did not do party politics," he said.

Former Social Democrat leader Martin Schulz told Germany's national news agency DPA, "Grenell isn't behaving like a diplomat, but instead like a right-wing colonial officer. Ambassadors are representatives of their states and not of political movements."

The choice of platform is potentially provocative too. On at least two occasions, Breitbart's Germany reporting has been exposed as misleading or even wholly false.

A report that a group of Muslim migrants had set fire to Germany's oldest church on New Year's Eve 2016 was later debunked by Dortmund's police. The church in question was not set on fire, nor was it Germany's oldest.

Last summer, Breitbart was forced to issue an apology after using a picture of German soccer player Lukas Podolski to illustrate a story about migrants traveling to Spain.

It is the second time in less than a week that a US ambassador has made public comments considered to be partisan. In an interview with The Times of Israel last week, David Friedman, US Ambassador to Israel, criticized the Democratic Party for failing to "create support within their constituency for Israel at the same levels that the Republicans have."

Former Congressman Ron Klein, chair of the Jewish Democratic Council of America, described the remarks in a statement as "truly unprecedented" and said that such "partisan behavior is damaging to our national interests and must stop immediately."

Retired US Ambassador Ronald Neumann told CNN that American diplomats normally stay out of anything that directly touches on particular political parties.

"This is not protocol or political correctness, but common sense," said Neumann, now the president of the American Academy of Diplomacy. "If the group you favor loses, the resulting government is not likely to look kindly on your interference."

Referring to Russian President Vladimir Putin, Neumann, a former ambassador to Afghanistan and Bahrain, said that the US "would be offended if Mr. Putin spoke publicly about his preference for a US political party. The reaction of others is the same."