(Adds trading, details on previous comments)
SAN FRANCISCO, Jan 22 (Reuters) - Activist investor Carl Icahn lashed out against Apple Inc again on Wednesday, saying the iPhone maker was doing shareholders a "great disservice" by refusing to boost its share buyback program.
The billionaire, who has repeatedly urged Apple to return more cash to shareholders, tweeted that he had bought $500 million more shares in the company in just the past two weeks, bringing his total investment to more than $3 billion.
Shares of the world's largest technology company climbed 1.4 percent to $556.71 in morning trading.
It was unclear how much stock the activist investor, who since August has tried to get Apple CEO Tim Cook to agree to a $150 billion buyback, has recently added to his portfolio. In a letter to Cook made public on Oct. 24, Icahn said he had increased his stake to 4.7 million shares.
In December, Icahn filed a shareholder proposal with Apple for a much smaller additional stock buyback plan of $50 billion, a major step back from his previous demands.
Apple is currently in the midst of returning $100 billion to shareholders, including a total share repurchase program of $60 billion.
"We feel (Apple's) board is doing great disservice to shareholders by not having markedly increased its buyback. In-depth letter to follow soon," Icahn said on Twitter.
"Since tweeting about our large position in $AAPL on Aug 13, when the stock was 468 per share, we've kept buying shares of this 'no brainer,"' Icahn added on his public profile.
Since Icahn first disclosed his slice of the U.S. company in August, Apple's shares have gained 12 percent.
Icahn was not immediately available for comment.
(Reporting by Edwin Chan; Editing by Bernadette Baum)