Billionaire investor Carl Icahn ramped up his battle with a rival hedge-fund manager Friday, reporting a 13 percent holding in Herbalife Ltd. and saying he would seek talks with the nutritional-supplements company.

Icahn is involved in a public dispute with hedge-fund manager William Ackman over whether Herbalife is a legitimate business.

Icahn, 76, said strategic alternatives for Herbalife may include taking it private, according to a filing with the U.S. Securities and Exchange Commission in which he reported his stake. Herbalife shares closed at $38.74, up 1.23 percent.

Further stock purchases or going private may increase Herbalife's share price and put pressure on Ackman, 46, who disclosed in December that his Pershing Square Capital Management Lp had sold short 20 million shares of Herbalife, calling the company a pyramid scheme.

In a short sale, an investor borrows stock and then sells the shares in anticipation of returning them at a lower price in the future. If stock rises instead, however, the short-selling investor will be forced to buy it back at a higher price than what he or she sold it for, resulting in a loss.

Icahn, speaking in a telephone interview with CNBC on Friday, said, "I'm doing this to make money, that's what I do. The fact that I don't like Ackman is, you could say, the strawberry on the ice cream." Icahn was speaking on the same CNBC show on which he and Ackman had a shouting match three weeks ago over Herbalife and other topics that became the talk of Wall Street.

Icahn said he sees "great value" in Herbalife and that Ackman had created an opportunity for him by pushing down the stock price. Herbalife slumped as low as $26.06 in December after Ackman said he was shorting the stock.

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Ackman remained firm in his beliefs about Herbalife.

"After 18 months of due diligence, we have concluded that it is a certainty that Herbalife is a pyramid scheme," Ackman said in an emailed statement.