Corporate raider Carl Icahn has taken a "large position" in Apple, calling the US tech giant "extremely undervalued".
Icahn, who in the midst of a fierce battle over computer maker Dell and has a history of taking stakes in firms to seek shakeups, announced his move on Twitter on Tuesday.
"We currently have a large position in APPLE," he tweeted. "We believe the company to be extremely undervalued. Spoke to (Apple chief executive) Tim Cook today. More to come."
In a second tweet moments later, Icahn said he would press Apple to increase its stock buyback, in which the company purchases its own shares in a bid to boost its value.
"Had a nice conversation with Tim Cook today. Discussed my opinion that a larger buyback should be done now. We plan to speak again shortly," Icahn tweeted.
Apple shares, which had been flat most of the day, jumped on the news and at 1852 GMT (0452 AEST on Wednesday) were up 3.7 per cent at $US484.69 ($A532.20).
Shares in Apple last year surged above $US700 but have been in a deep slump over concerns the maker of the iPhone and iPad has been losing its edge in innovation.
