SoftBank's billionaire Chief Executive Officer Masayoshi Son has been persistent in seeking a larger stake in the startup after investing in the last funding round, according to CNBC, which first reported the news.

The Microsoft-backed artificial intelligence start-up raised $500 million from the Japanese conglomerate, according to a media report, in a $6.6 billion funding round in October at a valuation of $157 billion.

OpenAI employees will have until December 24 to decide if they want to participate in the new funding round, a source told Reuters, adding that the stock's offer price aligns with the company's last funding round. The investment will come from SoftBank's Vision Fund 2, another source said.

--Reuters--