(Reuters) -ChatGPT-owner OpenAI is allowing its employees to sell roughly $1.5 billion worth of shares in a new tender offer to Japan's SoftBankGroup ( 9984.T , SFTBY ), two sources familiar with the matter told Reuters on Wednesday.

SoftBank's billionaire CEO 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 ( MSFT )-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 Dec. 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.

The sources declined to be named as they were not authorized to speak to the media. SoftBank and OpenAI declined to comment.

Son has been pushing to expand his conglomerate's exposure to the AI wave, taking a stake in OpenAI and acquiring chip startup Graphcore.

He recently said that he was saving up funds "so I can make the next big move," but did not provide any details about his investment plans.

OpenAI's meteoric rise in terms of product popularity and valuation has captured the world's imagination. Since the launch of ChatGPT, it has attracted 250 million weekly active users.

(Additional reporting by Shubham Kalia in Bengaluru; Editing by Abinaya Vijayaraghavan)