Co-founder and CEO Jeff Bezos has for the second successive month sold billions of dollars worth of Amazon shares.

Jeff Bezos sold almost US$3 billion in Amazon stock in five days last week, in what is being tipped as the co-founder and CEO’s biggest ever sale of shares in dollar terms.

Bezos, ranked by Forbes as the world’s richest person with $110.1 billion, told journalists at a space exploration conference in 2017 that he planned to sell about US$1bn worth of Amazon stock annually to fund Blue Origin, his rocket ship venture that is targeting its first crewed space flight this year. Bezos pledged US$2bn to launch a charity focused on early childhood education and homeless families last year.

Regulatory filings published on Monday did not give a reason for the sale. Amazon declined to comment beyond the filings, Financial Times reported.

According to the filings, Bezos sold nearly 532,000 shares on August 1 and 2 at prices ranging between US$1,800 and US$1,900 a share, worth about US$990m. He also donated 268 shares to an unnamed non-profit organisation.

The sale of shares came after the disclosed sale of more than 950,000 shares between July 29 and July 31 for around US$1,900 a share, for a total of about US$1.8bn.

Bezos has invested in Airbnb, healthcare company Grail, Whole Foods, Twitter, and Uber. In 2013, he bought The Washington Post for US$250 million. He is also one of the biggest landowners in the US. Bezos owns a 5.3-acre estate in Medina, Washington, valued at US$25 million, a former textile museum turned townhouse in Washington DC that cost US$23 million, and three apartments in New York City amounting to a total value of US$17 million. He also owns a US$25 million mansion in Beverly Hills and bought a US $12.9 million four-bedroom property next door, last month. His property portfolio also includes a 30,000-acre ranch in Texas, which is the base for Blue Origin.

Bezos’s remaining 12 per cent stake in Amazon is worth around US$102bn at Monday’s closing price of US$1,765.13 per share.

Amazon shares are up about 20% since the beginning of the year.