OpenAI announced on Tuesday its co-founder Sam Altman will return as CEO, days after he was fired by the board.

"We have reached an agreement in principle for Sam to return to OpenAI as CEO with a new initial board of Bret Taylor (Chair), Larry Summers, and Adam D'Angelo," it said in a post on X, formerly Twitter.

OpenAI's board sacked Altman last week, with Microsoft's CEO Satya Nadella announcing soon after he was hiring Altman to run a new artificial intelligence research team.

But in the fast-moving sequence of events, Altman said Tuesday he had the support of Nadella for his return as OpenAI's CEO.

"With the new board and w satya's support, i'm looking forward to returning to openai, and building on our strong partnership with (Microsoft)," he wrote on X.

Hundreds of staff at OpenAI threatened to quit following Altman's dismissal.

In a letter released to media, the vast majority of the company's 770-strong staff suggested they would follow Altman unless the board responsible for his departure resigned.

Among the signatories was co-founder Ilya Sutskever, OpenAI's chief scientist and a member of the four-person board who pushed Altman out.

"I deeply regret my participation in the board's actions," Sutskever wrote on X on Monday. "I never intended to harm OpenAI."

OpenAI had appointed Emmett Shear, a former chief executive of Amazon's streaming platform Twitch, as its new CEO despite pressure from Microsoft and other major investors to reinstate Altman.