ChatGPT creator OpenAI will allow its employees to sell shares worth roughly $1.5 billion to Japanese tech investor SoftBank, CNBC reported on Wednesday.

The purchases in a tender offer will allow SoftBank to increase its stake in OpenAI and permit current and former OpenAI employees to cash out their shares, two anonymous sources told the US media outlet.

A person familiar with the matter told AFP that the report was correct.

Employees have until December 24 to decide if they want to participate in the new tender offer, CNBC quoted one of its sources as saying.

The deal was instigated by SoftBank's billionaire CEO Masayoshi Son, who was persistent in asking for a larger stake in the start-up after putting $500 million into OpenAI's last funding round, CNBC added.

SoftBank was not immediately available for comment.

US-based OpenAI rose to global prominence after it released popular generative-AI chatbot ChatGPT in 2022.

Its CEO Sam Altman catapulted the company to a staggering $157 billion valuation in a recent round of fundraising that included Microsoft, SoftBank and chipmaker Nvidia as investors.

Son, 67, made his name with successful early investments in Chinese ecommerce titan Alibaba and internet pioneer Yahoo, but has also bet on catastrophic failures such as WeWork.

Son has repeatedly said that "artificial superintelligence" will arrive in a decade, bringing new inventions, new medicine, new knowledge and new ways to invest.