SAN FRANCISCO: Activist investor Carl Icahn has raised his stake in Apple as he escalates his campaign to pressure the company into buying back more of its stock while the shares remain stuck far below their peak price.

In a Wednesday post on his Twitter account, Icahn revealed that he has poured another $500 million into Apple stock during the past two weeks. He already owned about 4.7 million Apple Inc. shares worth more than $2.5 billion.

Given the stock's trading range during the past two weeks, Icahn probably picked up another 800,000 to 1 million shares. That would still leave his total stake in the iPhone and iPad maker below 1 percent.

Icahn is urging Apple to spend $50 billion buying back its own stock during the current fiscal year ending in September.

That target would require Apple's eight-member board to up the ante on a $60 billion stock buyback program adopted nine months ago. Apple still had $37 billon available to spend under that program entering October. Regulatory filings indicate Apple spent about $5 billion of that amount buying back its stock during the first three months of this fiscal year.

The company is expected to provide a specific figure on its stock buybacks Monday when it releases its latest quarterly earnings.

Apple's stock gained $2.44 to $551.51 Wednesday. The shares peaked at $705.07 in September 2012, then sank on concerns about intensifying competition from other mobile device makers and the lack of a breakthrough product since Apple's former CEO Steve Jobs died in 2011. Apple's stock has surged by nearly 20 percent since last August when Icahn announced his initial stake in the company.

Despite having minimal financial leverage, Icahn is still hoping his track record for prodding companies to make moves that have increased their companies' stock prices will persuade Apple's board of directors to follow his advice.

"At the risk of being immodest, I think I have one hell of a good record on telling you how to use your money or how to invest it," Icahn said during a Wednesday interview on CNBC. "Why don't they listen to some extent?"

Icahn told CNBC he will release a letter elaborating on his ideas within the next day or two.

Apple CEO Tim Cook met with Icahn during a dinner in New York last fall, but so far the Cupertino, Calif., company hasn't placated the acerbic billionaire. Echoing previous remarks, Icahn told CNBC it's a "no-brainer" to buy Apple stock now because he considers the current prices to still be a bargain.

All of Apple's shareholders will have a chance to weigh in how the board should respond to Icahn at the company's annual meeting on Feb. 28. Icahn is making his stock-buyback pitch in a non-binding proposal that Apple's board is opposing. The board says it is still meeting with investors as it mulls way to return more of Apple's cash to shareholders.