Aggressive US investor Carl Icahn has increased pressure on Apple management by buying more shares in the company and tweeting on Wednesday that it is time for the company to increase its buyback.

“We feel $APPL board is doing great disservice to shareholders by not having markedly increased its buyback,” Icahn said on Twitter.

Icahn Enterprises has purchased $500 million in Apple shares in the last two weeks, bringing its investment to more than $3 billion, Icahn tweeted.

Icahn’s share is worth only one-tenth of 1 per cent of Apple’s total estimated value of $500 billion. But Icahn’s strategy is to leverage small holdings in companies to build coalitions among shareholders that receive large media coverage.

In October, Icahn and other large shareholders forced computer maker Dell to give up plans to sell the company. Instead, company founder Michael Dell bought back the company by sweetening the offer with an additional half a billion dollars.

Icahn has been pushing for Apple to increase its share buyback programme to $50 billion in 2014.

But the company in December rebuffed his push, noting it already has a long-term share buyback programme worth $100 billion, and spent $23 billion on it in 2013.

The next shareholders’ meeting is on February 28. A buyback would further increase Apple’s share price to the benefit of shareholders.