The billionaire all-star investor disclosed the move in an interview with CNBC, telling the broadcaster that the stock purchases were bought by “one of the fellows in the office that manage money.”

Buffett hired investment managers Todd Combs and Ted Weschler in 2010 and 2012 to manage stock portfolios of more than US$13 billion for Berkshire.

Stock picking has been a key aspect of Berkshire’s success, helping to establish the folksy Buffett as one of the richest people in the world, although he has in the past been reticent about diving into the tech sector.

Berkshire also has subsidiaries in a broad range of sectors, including insurance, freight rail transportation and electric utilities.

Buffett’s stock picking has tended to favour nuts-and-bolts sectors such as banking, airlines and food companies.

He holds a stake in Apple and had a sizeable investment in IBM at one point but has generally refrained from investing in many of the biggest technology companies that have become powerhouses in the 21st Century, including Google, Facebook and – until now – Amazon.

Buffett previously has lamented not getting into Amazon at earlier stage, blaming “stupidity” for missing the opportunity in a 2017 interview.

Amazon rose 2.5% to US$1959.15 in opening trading.