The Berlin government declined to give an official reaction to a video circulating on the internet which showed members of the "alt-right" movement, a grouping that includes neo-Nazis, white nationalists and anti-Semites, meeting on Saturday in Washington a few blocks from the White House.
But one senior official close to Merkel described the video - which shows a speaker shouting "Hail Trump" and some audience members making the Nazi salute - as "repulsive and worrying".
"I don't think this is Trump's ideology because he seems to be largely free of ideology. But these people are riding on his coattails. We are watching this very closely," the official said, requesting anonymity.
Yair Lapid, a member of the foreign affairs and defense committee in the Israeli Knesset, called the video "sickening" and "intolerable".
"One of the greatest mistakes humanity ever made was a failure to recognizes the danger of fascism early enough and tackle it head on," said Lapid. "We cannot let history repeat itself."
A spokesman for the Trump-Pence transition team said on Monday that Trump "continued to denounce racism of any kind" and was elected to be "a leader for every American."
Trump, who has been active on Twitter in recent days, has not commented directly on the meeting himself. It came days after he outraged many Democrats, rights activists and minority groups by appointing Steve Bannon, former head of a website linked to the alt-right, as his chief White House strategist.
In the video, taken inside the conference and published by The Atlantic, Richard Spencer, a leader of the "alt-right" movement, says America belongs to white people, who he describes as "children of the sun". He denounces the movement's critics as "the most despicable creatures who ever walked the planet".
"Hail Trump, hail our people, hail victory!" Spencer shouts at one point as some members of the audience raise their arms in the Nazi salute.
The gathering on Saturday drew protesters who blocked traffic around the Ronald Reagan Building, a federally owned conference center in downtown Washington for both public and private use.
David Harris, CEO of the American Jewish Committee (AJC) in New York, said fringe groups espousing anti-Semitism and targeting minorities had emerged from the U.S. presidential campaign with a "vigour" that has not been seen in decades.
"President-elect Trump issued a statement (about the Saturday meeting) but, assuming this group endures, will need to continue to find other opportunities to make crystal clear that he is the president of all Americans and condemns the alt-right and what it stands for," Harris told Reuters in response to written questions about the meeting.
In Germany, which has spent the past 70 years atoning for its Nazi past, using the "Heil Hitler" salute and other Nazi symbols is illegal and can result in a prison sentence of up to six months. Other European countries including Austria and France have similar laws.
A second German government official said Trump's decision to bring Bannon into the White House showed he was "not willing to forgo the movement and mobilization of anger and resentment" that swept him to the presidency.
Guy Verhofstadt, former Belgian prime minister and a member of the European Parliament, accused Bannon on Tuesday of seeking to influence elections in France and Germany next year with the launch of new Breitbart News websites in Europe.
American allies face a delicate balancing act when reacting to incidents and events like the one on Saturday, which have no direct link to Trump but seem fueled by his campaign, in which he called Mexicans rapists and floated the idea of registering Muslims like the Nazis did with Jews.
David Keyes, foreign media spokesman for the Israeli Prime Minister Benjamin Netanyahu, said in response to a Reuters query about the Washington meeting: "Prime Minister Netanyahu condemns anti-Semitism everywhere and appreciates President-elect Trump's denunciation of all forms of racism."
Pinchas Goldschmidt, president of the Conference of European Rabbis, told Reuters that the United States appeared to be "veering away" from its own moral standards and might need to reexamine its stance on free speech.
"If words like this were used in Germany or Austria or France the people would have gotten in trouble with the law," he said, referring to the Washington meeting.
"Social media has created huge change. It has empowered and amplified the voices on the fringes. There may be a need in the United States to consider changes or limits to free speech to address this."