The lawsuit filed late Thursday said Altman and OpenAI's co-founder Greg Brockman originally approached Musk to make an open source, non-profit company.

The Microsoft-backed company's focus on making money breached that contract, lawyers for Musk said in the lawsuit filed in San Francisco. They added that the company had kept the design of GPT-4, its most advanced AI model, "a complete secret."

OpenAI, Microsoft and Musk did not immediately respond to Reuters requests for comment.

Musk co-founded OpenAI in 2015, but stepped down from its board in 2018. He also runs electric vehicle maker Tesla and rocket maker SpaceX and bought Twitter for $44 billion in October 2022.

Last year serial entrepreneur Altman was fired by OpenAI's former board which said it was trying to defend the company's mission to develop AI that benefits humanity. A few days later, Altman returned to the company with a new initial board.

OpenAI is planning to appoint several new board members in March, the Washington Post reported on Thursday.

ChatGPT, the chatbot from OpenAI, became the fastest-growing software application in the world within six months of its launch in November 2022. It also sparked the launch of rival chatbots from Microsoft, Alphabet and a bevy of startups that tapped the hype to secure billions in funding.

Since its debut, ChatGPT has been adopted by companies for a wide range of tasks from summarizing documents to writing computer code, setting off a race amongst Big Tech companies to launch their own offerings based on generative AI.