Icahn's comments made on Twitter suggested tensions rising with the iconic California maker of iPads and iPhones, after having met chief executive Tim Cook for dinner last year to press his case to do more for shareholders.
"Having purchased USD 500 million more USD AAPL shares in the last two weeks, our investment has crossed the USD 3 billion mark yesterday," Icahn said in a tweet.
Moments later, he said in another tweet that Apple "is doing great disservice to shareholders by not having markedly increased its buyback. In-depth letter to follow soon."
But the corporate raider, who unveiled his investment in Apple last August, is continuing to press for an increased stock buyback which could boost the value of shares.
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Last year, Apple agreed to a plan to return some $100 billion to shareholders including $60 billion in buybacks.
Icahn, however, has talked about a bigger share buyback, at one point proposing USD 150 billion. In December, he said he would make a formal proposal to the tech giant's board of directors but scaled back his idea for a USD 150 billion program.
Apple stock was up 1.47 per cent at USD 557.12 at 1620 GMT.