The White House has accused Elon Musk of repeating a "hideous lie" about Jewish people, after the X owner appeared to respond approvingly to an antisemitic post on the platform.
On Wednesday, Mr Musk replied to a post sharing an antisemitic conspiracy theory, calling it "actual truth".
Mr Musk has denied that the post was antisemitic.
But a White House spokesman said his endorsement of the post, which drew anger online, was "unacceptable".
"We condemn this abhorrent promotion of antisemitic and racist hate in the strongest terms," said White House spokesperson Andrew Bates.
He noted that the post Mr Musk was responding to referred to a conspiracy theory that motivated the man who killed 11 people at a Pittsburgh synagogue in 2018.
"It is unacceptable to repeat the hideous lie behind the most fatal act of antisemitism in American history at any time, let alone one month after the deadliest day for the Jewish people since the Holocaust," Mr Bates said, referring to the 7 October Hamas assault against Israel.
X Chief Executive Linda Yaccarino wrote in an earlier tweet that the company has been "extremely clear about our efforts to combat antisemitism and discrimination. There's no place for it anywhere in the world - it's ugly and wrong".
On Wednesday, Mr Musk responded with his "truth" comment to a post that accused Jewish communities of pushing "hatred against whites" and which included anti-immigrant sentiments.
It appeared to be an endorsement of a racist and antisemitic conspiracy theory known as "white genocide," which argues that Jewish people systematically plot to encourage immigration of "non-white" people to Western countries in order to "eliminate" the white race.
The original post that Mr Musk responded to "is using specific language that has been used in the past to justify violent attacks on synagogues," Zahed Amanullah, senior fellow at the London-based Institute of Strategic Dialogue, told the BBC.
The conspiracy theory motivated a mass murderer who entered the Tree of Life synagogue in Pittsburgh in 2018 and shot dead 11 worshippers.
Mr Musk denies he is antisemitic and later said his comments referred not to all Jewish people but to groups like the Anti-Defamation League (ADL) and other unspecified "Jewish communities".
ADL Chief Executive Jonathan Greenblatt posted: "At a time when antisemitism is exploding in America and surging around the world, it is indisputably dangerous to use one's influence to validate and promote antisemitic theories."
The controversy over antisemitism comes as some organisations have stopped buying ads on X, formerly known as Twitter, citing extremist content on the social network.
IBM stopped its ad spending after a report from a left-wing media watchdog said its content was placed next to posts praising Adolf Hitler and Nazism. Apple later said it too would halt ad buys on the platform, Axios reported.
X told the BBC on Thursday that ads are not deliberately placed next to extremist content, that the Nazi-promoting accounts will not earn money from advertising and that specific posts will be labelled "sensitive media".
Separately, the European Commission has asked its departments to stop buying ads on X because of concerns over misinformation in relation to the Israel-Hamas war, according to a report by Politico.
On the platform on Friday, Mr Musk did not directly address his own statements but criticised Media Matters and responded in support of other posts critical of IBM and "media".
The billionaire has on several occasions repeated conspiracy theories and has also lashed out at social media watchdogs - including the ADL and other groups - for criticising his content moderation changes at X.
X claims that it has stronger brand safety controls than other social networks and that hate speech and extremism has fallen on the platform despite large cuts to the company's safety team. Several outside groups disagree with the company's assessment and say that extremism and hate speech have increased under Mr Musk's leadership.
Earlier this year Mr Musk threatened to sue the ADL, claiming it was "trying to kill this platform by falsely accusing it & me of being anti-Semitic". He blames pressure groups, rather than misinformation and extremist posts, for a sharp drop in advertising revenue since his takeover.
While he has not carried through with his threat against the ADL, the company has sued another research and campaign group, the Center for Countering Digital Hate.
On Thursday, CCDH filed a motion to dismiss the lawsuit under California's anti-SLAPP - "Strategic Lawsuits Against Public Participation" - law, calling the X suit "an attempt to censor, intimidate, and silence".
With reporting by Chris Vallance.