The White House said that Elon Musk was spreading a “hideous lie” about Jews after the CEO of SpaceX seemed to support an antisemitic post on the site.

On Wednesday, Mr. Musk responded to a post that shared an anti-Semitic conspiracy theory by saying that it was “actual truth.”

Mr. Musk has said that the post was not anti-Semitic.

But a White House official said it was “unacceptable” that he supported the post, which made people angry online.

Andrew Bates, a spokesman for the White House, said, “We strongly condemn this disgusting spread of racist and antisemitic hate.”

He said that the post Mr. Musk was reacting to was about a conspiracy theory that the person who killed 11 people at a Pittsburgh synagogue in 2018 was inspired by.

Mr. Bates said, “It is unacceptable to repeat the hideous lie behind the deadliest act of antisemitism in American history at any time, let alone one month after the deadliest day for the Jewish people since the Holocaust.” He was speaking to the attack by Hamas on Israel on October 7.

In a previous tweet, X CEO Linda Yaccarino said that the company has been “extremely clear about our efforts to fight antisemitism and discrimination.” It has no place in the world; it’s ugly and wrong.

Mr. Musk’s “truth” comment on Wednesday was in response to a post that said Jewish groups were spreading “hatred against whites” and anti-immigrant feelings.

Apparently, they agreed with the racist and antisemitism theory of “white genocide,” which says that Jewish people plan to get “non-white” people to move to Western countries so that they can “eliminate” the white race.

A senior fellow at the London-based Institute of Strategic Dialogue told the BBC that the original post that Mr. Musk replied to “uses specific language that has been used in the past to justify violent attacks on synagogues.”

The mass killer who went into the Tree of Life synagogue in Pittsburgh in 2018 and shot and killed 11 people there was inspired by the conspiracy theory.

A later statement said that Mr. Musk’s words were not about all Jews, but about the Anti-Defamation League (ADL) and other “Jewish communities” that were not named.

Jonathan Greenblatt, CEO of ADL, wrote on Facebook, “At a time when antisemitism is on the rise in America and around the world, it is clearly dangerous to use your power to support and spread antisemitic ideas.”

Some groups have stopped buying ads on X, which used to be known as Twitter, because of extremist content on the social network. This comes at the same time as the antisemitism issue.

A left-wing media group said that IBM’s ads were shown next to posts that supported Adolf Hitler and Nazism. This caused IBM to stop spending money on ads. Axios reported that Apple later said it would also stop buying ads on the app.

X told the BBC on Thursday that ads are not put next to extreme content on purpose, that accounts that promote Nazism will not make money from ads, and that certain posts will be marked as “sensitive media.”

A story from Politico says that the European Commission has asked its departments to stop buying ads on X because of worries about false information about the war between Israel and Hamas.

On Friday, Mr. Musk didn’t directly address what he had said, but he did criticize Media Matters and back up other posts that were negative about IBM and the “media.”

The billionaire has spread conspiracy theories more than once and has criticized social media watchdogs, such as the ADL and other groups, for criticizing the changes he made to how material is moderated at X.

X says that its brand safety controls are better than those on other social networks and that the amount of hate speech and violence on the platform has gone down, even though the company has cut its safety team by a lot. A number of outside groups don’t agree with the company’s opinion and say that Mr. Musk’s leadership has led to more extremism and hate speech.

Early this year, Mr. Musk said the ADL was “trying to kill this platform by falsely accusing it and me of being anti-Semitic,” and he meant to sue them. He says that pressure groups, not false information and extremist posts, are to blame for a big drop in ad income since he took over.

He made a threat against the ADL, but it hasn’t come true yet. Instead, the company has sued the Center for Countering Digital Hate, a study and advocacy group.

California has a rule against “Strategic Lawsuits Against Public Participation” (SLAPPs), and on Thursday, CCDH asked the court to throw out the lawsuit. They said the X suit was “an attempt to censor, intimidate, and silence.”