"Apple is behaving in a manner that makes it impossible for any AI company besides OpenAI to reach #1 in the App Store, which is an unequivocal antitrust violation," Musk said in a post on his social network X on Monday, without providing evidence to back his claim.

"xAI will take immediate legal action," he added, referencing his own artificial intelligence company.

X users responded by pointing out that DeepSeek AI out of China hit the top spot in the App Store early this year, and Perplexity AI recently ranked number one in the App Store in India.

DeepSeek and Perplexity compete with OpenAI and Musk's startup xAI.

Both OpenAI and xAI released new versions of their AI assistants, ChatGPT and Grok, in the past week.

App Store rankings on Tuesday listed ChatGPT as the top free iPhone app with Grok in fifth place.

Apple did not immediately respond to a request for comment.

Factors going into App Store rankings include user engagement, reviews, and the number of downloads.

OpenAI and Apple in June of last year announced an alliance to enhance iPhones and other devices with ChatGPT features.

ChatGPT-5 rolled out free to the nearly 700 million people who use it weekly, OpenAI said in a briefing with journalists last week.

Tech industry rivals Amazon, Google, Meta, Microsoft and xAI have been pouring billions of dollars into artificial intelligence since the blockbuster launch of the first version of ChatGPT in late 2022.

Chinese startup DeepSeek shook up the AI sector early this year with a model that delivers high performance using less costly chips.

OpenAI in April of this year filed counterclaims against multi-billionaire Musk, accusing its former co-founder of waging a "relentless campaign" to damage the organization after it achieved success without him.

In legal documents filed at the time in northern California federal court, OpenAI alleged Musk became hostile toward the company after abandoning it years before its breakthrough achievements with ChatGPT.

The lawsuit was another round in a bitter feud between the generative AI (genAI) start-up and the world's richest man, who sued OpenAI last year, accusing the company of betraying its founding mission.

In its countersuit, the company alleged Musk "made it his project to take down OpenAI, and to build a direct competitor that would seize the technological lead -- not for humanity but for Elon Musk."

Musk founded his own genAI startup, xAI, in 2023 to compete with OpenAI and the other major AI players.