Washington DC - Elon Musk's X sued an advertising group and several large corporations on Tuesday accusing them of causing billions of dollars of losses by "illegally" boycotting the social media platform.

"We tried peace for 2 years, now it is war," the billionaire founder of Tesla and SpaceX said on X, which he acquired in late 2022.

The antitrust suit, filed in a federal court in Texas, targets the World Federation of Advertisers (WFA), Unilever, Mars, CVS Health, and Orsted, a Danish energy company.

It accuses WFA, through an initiative known as the Global Alliance for Responsible Media (GARM), of conspiring with the companies and others to "collectively withhold billions of dollars in advertising revenue" from X, formerly Twitter.

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A number of advertisers left Twitter following Musk's takeover amid concerns about the level of content moderation under the new ownership and Musk's own controversial musings on the site.

X CEO Linda Yaccarino, in a video posted on the platform on Tuesday, said X was the victim of a "systematic illegal boycott."

"They conspired to boycott X which threatens our ability to thrive in the future," Yaccarino said.

"No small group of people should be able to monopolize what gets monetized."

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X is seeking a jury trial and unspecified damages.

According to The New York Times, citing internal company documents, X earned $114 million in revenue in the US in the second quarter of this year, down 25% from the first quarter and down 53% from the same period last year.

The lawsuit was filed one day after Musk filed a since-dropped lawsuit in California against OpenAI, accusing its co-founders Sam Altman and Greg Brockman of betraying the artificial intelligence company's founding mission.

Musk invested in the San Francisco-based OpenAI in 2015 but left three years later.