(Reuters) -Elon Musk threatened on Saturday to sue media watchdog Media Matters and those who attacked his social media platform X, following moves by several large U.S. companies to halt advertising on the site after being promoted alongside antisemitic content.

Musk and X have been under a microscope all week for antisemitic and racist content that has proliferated on the site since he purchased it in 2022.

Liberal watchdog group Media Matters for America said earlier this week that it found ads from (NYSE:), Apple (NASDAQ:) and others were placed alongside content promoting Adolf Hitler and the Nazi Party.

Musk on Wednesday endorsed an antisemitic post on X that falsely claimed members of the Jewish community were stoking hatred against white people, drawing sharp condemnation, including from the White House.

"The split second court opens on Monday, X Corp will be filing a thermonuclear lawsuit against Media Matters and all those who colluded in this fraudulent attack on our company," Musk wrote in a post on X, without naming any other parties.

Numerous companies suspended ads in the last two days, including IBM, Disney, Warner Bros Discovery (NASDAQ:) and Comcast (NASDAQ:), Lions Gate Entertainment and Paramount Global. Axios reported that Apple, the world's largest company by market value, would do the same.

"This week Media Matters for America posted a story that completely misrepresented the real experience on X, in another attempt to undermine freedom of speech and mislead advertisers," a statement posted by Musk said. He accused Media Matters of creating an alternative account designed to "misinform advertisers" about their posts.

Media Matters on Saturday said Musk was a "bully" who threatens "meritless lawsuits".

"Musk admitted the ads at issue ran alongside the pro-Nazi content we identified," Media Matters President Angelo Carusone said in a statement.

"If he does sue us, we will win."

Musk has threatened legal action against other parties in the past, most specifically the Anti-Defamation League, a nonprofit organization that fights antisemitism, blaming it for X's loss of ad revenue. He has not yet sued the ADL, however.

Advertisers have fled the site since Musk bought it in October 2022 and reduced content moderation, resulting in a sharp rise in hate speech, according to civil rights groups.

The White House on Friday condemned Musk's endorsement of what it called a "hideous" antisemitic conspiracy theory, and accused Musk of an "abhorrent promotion of antisemitic and racist hate" that "runs against our core values as Americans".

Musk is also CEO of electric carmaker Tesla (NASDAQ:), which has been hit by several lawsuits that allege rampant racial or sexual harassment of workers.

Antisemitism has been on the rise in recent years in the United States and worldwide. Following the outbreak of war between Israel and Palestinian Islamist group Hamas, which attacked Israel on Oct. 7, antisemitic incidents in the United States rose by nearly 400% from the year-earlier period, the ADL said.