NEW YORK (Feb 28): Berkshire Hathaway Inc. increased its stake in iPhone maker Apple Inc. to about 133 million shares, Chairman Warren Buffett told CNBC.

That’s more than twice as much as Berkshire held as of Dec. 31, the billionaire told the cable network in an interview Monday. The stake is valued at more than US$18 billion ($25.3 billion), based on Friday’s closing price of US$136.66.

Berkshire has been accelerating stock purchases in recent months, adding to stakes in the largest U.S. airlines and Apple. Buffett said the 2017 Apple purchases were made before the company posted quarterly earnings on Jan. 31, a report that pushed shares higher than he wanted to pay.

“Apple strikes me as having quite a sticky product and an enormously useful product to people that use it, not that I do,” Buffett said, praising Tim Cook, the technology company’s chief executive officer. “He’s been very intelligent about capital deployment.”

Apple climbed 0.3% to US$137.10 in early trading at 7:32 a.m. in New York. That compares with US$105.26 at the end of 2015. Berkshire disclosed a stake in the first half of 2016 and increased the holding later that year.

Buffett said he accumulated about 123 million of the Apple shares, and that one of his deputies acquired the rest, without identifying the investment manager. Berkshire hired Todd Combs in 2010 and Ted Weschler in 2011 to help pick stocks and eventually assume his role managing the company’s investment portfolio.

Buffett also discussed the airline bets, saying that one of his deputies oversees a stake in American Airlines Group Inc., “and I have the other three.” Berkshire also holds stock in Delta Air Lines Inc., Southwest Airlines Co. and United Continental Holdings Inc., with stakes of more than 7% in each company.

“We don’t want to go over 10%, virtually on any stock. It complicates life for us,” Buffett said. “We do it occasionally, but it’s a big decision to make, whether to go over 10%.”

Weschler invested in the airline industry before joining Berkshire and took a fresh look at the business after American CEO Doug Parker gave an investor presentation in March, people familiar with the matter said earlier this year.