CUPERTINO, Calif.-- Apple Inc. has bought $14 billion of its own shares in the two weeks since reporting financial results that disappointed Wall Street, Chief Executive Tim Cook said in an interview.

Mr. Cook said Apple was "surprised" by the 8% decline in its shares on Jan. 28, the day after it reported lower iPhone sales than projected and warned that revenue in the current quarter might decline from the same period a year ago. Mr. Cook said he wanted to be "aggressive" and "opportunistic."

With the latest purchases, Mr. Cook said Apple had bought back more than $40 billion of its shares over the past 12 months, which Mr. Cook said was a record for any company over a similar span.

"It means that we are betting on Apple. It means that we are really confident on what we are doing and what we plan to do," said Mr. Cook, speaking in a conference room at the company's corporate headquarters here. "We're not just saying that. We're showing that with our actions."

Those purchases are part of Apple's previously disclosed plan to repurchase $60 billion of its own shares, Mr. Cook said. He said Apple bought $12 billion of the shares through an "accelerated" repurchase program, and $2 billion on the open market. He said Apple plans to disclose "updates" to its buyback program in March or April.

The revelation about the recent share purchases comes a few weeks before Apple's Feb. 28 shareholder meeting, where activist investor Carl Icahn is pressuring the company to be more aggressive with its $160 billion cash pile.

Mr. Icahn, who owns roughly $4 billion in Apple shares, is asking shareholders to vote on his proposal that Apple buy an additional $50 billion of its own shares by the end of September, above its current plan.

Responding to Mr. Cook's statement about his confidence in Apple, Mr. Icahn said, "So am I."

Apple's Mr. Cook said he wants to "be able to adjust for the long-term interest of the shareholders, not for the short-term shareholder, not for the day trader.

"We may see a huge company tomorrow that we want to acquire or something may happen in the stock market that's unpredictable," he continued.

Historically, Apple hasn't made big acquisitions; the company has never spent more than $1 billion on a single deal. Mr. Cook said Apple has bought 21 companies over the past 15 months.

That track record is a contrast to Google Inc., which has been snapping up companies across the technology landscape, most notably its $3.2 billion acquisition of Nest, a maker of connected home devices founded by former Apple employees.

The Apple CEO said its history of opting for smaller deals doesn't mean that the company won't pull the trigger on a big acquisition if it makes sense.

"We've looked at big companies," said Mr. Cook. "We have no problem spending 10 figures for the right company, for the right fit that's in the best interest of Apple in the long-term. None. Zero."

Apple is fighting the perception that its best days are behind it. Revenue growth has dropped to less than 10% in the last three quarters, while net income fell 11% in the past fiscal year--its first annual decline in more than a decade.

An often-cited factor is that the company hasn't broken into a new product category--as it did with the iPod and iPhone--since introducing the iPad in 2010.

Thursday, Mr. Cook reiterated that Apple plans to enter a new category this year. Apple watchers are speculating about wearable devices or a new television platform.

"There will be new categories. We're not ready to talk about it, but we're working on some really great stuff," Mr. Cook said. When asked whether a new product category could mean an improvement on an existing product like an iPad Air, a lighter version of its tablet computer, or new services such as mobile payments, Mr. Cook declined to comment.

He said that anyone "reasonable" would consider what Apple is working on as new categories.

Mr. Cook said he still considers Apple to be a "growth company" with significant opportunities to expand the position of existing products including the iPhone.

Apple has been losing market share in smartphones as the market grows. In 2013, Apple accounted for 15.5% of all smartphones shipped world-wide, down from 19.4% in 2012, according to Strategy Analytics. By comparison, manufacturers running Google's Android operating system accounted for 78.9% in 2013, up from 68.8% a year earlier.

Mr. Cook has said repeatedly that Apple aims to produce the best phones, not necessarily the most. That has fueled concerns that the iPhone will become a niche product, much as Apple's Macintosh was a niche product during the personal-computer era.

Mr. Cook said Apple has the largest or second-largest market share for true smartphones in nearly every country; he excluded smartphones that are used as feature phones because of their limitations.

"In the other places where we are number two, I'd rather be number one. And you can bet that we're working on that," said Mr. Cook.

He said his statement that Apple doesn't aim to make the most phones has been misunderstood.

"I don't view that as being satisfied with being small or however you want to define it," he said. "I just want to say that the macro thing for us is making a great product and we must do that. If we can't do that, we're not going to force ourselves to hit a price point that makes us produce a product that we're not proud of because we lose who we are in that. We're not going to do that."

Write to Daisuke Wakabayashi at Daisuke.Wakabayashi@wsj.com

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