San Francisco, California - Elon Musk has launched a legal case against OpenAI, the AI firm he helped to set up in 2015, accusing its leaders of a "betrayal" of its founding mission.

The tycoon, who left OpenAI in 2018, argued in documents filed in a San Francisco court late on Thursday that firm was always intended as a non-profit entity.

But he said recent boardroom changes meant OpenAI was now effectively a subsidiary of software giant Microsoft.

Musk has made similar accusations in the past, with both OpenAI and Microsoft denying them.

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OpenAI became the next big thing in 2022 with the release of its chatbot ChatGPT, and it has since also developed image and video generating tools that are seen as the leaders in their field. Microsoft, a major investor in OpenAI since 2019, poured billions more into the firm last year.

And the giant firm stepped in when OpenAI's board fired CEO Sam Altman in November last year, hiring him and offering to house any staff members who were unhappy with his ousting.

The OpenAI board later climbed down, Altman was reinstated, and several board members were replaced.

OpenAI started life as a non-profit dedicated to developing "artificial general intelligence" (AGI), a vague term loosely defined as a kind of AI that would far outstrip human capabilities on all measures of intelligence.

The idea was for OpenAI to guarantee that such technology would be safe for humanity.

But Musk's legal case said this founding principle had been "turned on its head".

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New members of OpenAI's board had no AI expertise and had been "hand-picked by Mr Altman and blessed by Microsoft", the filing alleged.

"To this day, OpenAI Inc's website continues to profess that its charter is to ensure that AGI 'benefits all of humanity'," the filing stated.

"In reality, however, OpenAI Inc has been transformed into a closed-source de facto subsidiary of the largest technology company in the world: Microsoft."

Since leaving OpenAI, Musk has joined the chorus of critics warning that superintelligence could spell the end for humanity.

At the same time, he also launched his own AI firm, xAI, last year and said he wanted to raise $1 billion from investors.