(Reuters) - Elon Musk's social media platform X on Tuesday sued a global advertising alliance and several major companies, including Mars and CVS Health ( CVS ), accusing them of unlawfully conspiring to boycott the site and causing it to lose revenue.

X filed the lawsuit in federal court in Texas against the World Federation of Advertisers, Unilever ( UL ) and Danish renewable energy company Orsted, in addition to Mars and CVS Health ( CVS ).

The lawsuit said advertisers, acting through a World Federation of Advertisers initiative called Global Alliance for Responsible Media, collectively withheld "billions of dollars in advertising revenue" from X, previously known as Twitter.

It said they acted against their own economic self interests in a conspiracy against the platform that violated U.S. antitrust law.

The World Advertising Federation, Unilever ( UL ), Mars, CVS Health ( CVS ) and Orsted did not immediately respond to requests for comment.

In a statement on Tuesday about the lawsuit, X's chief executive Linda Yaccarino said "people are hurt when the marketplace of ideas is constricted. No small group of people should monopolize what gets monetized."

Ad revenue at X slumped for months after Musk bought the company in 2022. Brands had been wary of rapid changes initiated under Musk's ownership.

The advertising group launched the responsible media initiative in 2019 to "help the industry address the challenge of illegal or harmful content on digital media platforms and its monetization via advertising."

X said in its lawsuit that it has applied brand-safety standards that are comparable to those of its competitors and that "meet or exceed" measures specified by the Global Alliance for Responsible Media.

The lawsuit said X has become a "less effective competitor" in the sale of digital advertising.

X is seeking unspecified damages and a court order against any continued efforts to conspire to withhold ad dollars.