Herbalife shares rose 4 percent in extended trading, reversing course after Icahn said he had bought 2.3 million shares on Friday.

"Completely contrary to what Bill Ackman stated on television today, I have never given Jefferies an order to sell any of our Herbalife shares," Icahn said in a statement after market close.

Ackman, who unveiled a $1 billion short bet against Herbalife in 2012, said on CNBC earlier in the day that Icahn's planned stake sale would accelerate the company's downfall.

"I continue to believe in Herbalife," Icahn said, disclosing a 20.78 percent stake in the company.

He held 18.32 percent of Herbalife as of July 15.

Icahn and Ackman have opposing bets on Los Angeles-based Herbalife. Ackman for years has accused Herbalife of running a pyramid scheme and bet that the stock would fall to zero.

The duo even became embroiled in a public war of words, with Icahn famously calling Ackman a "liar" and a "crybaby" in a CNBC interview in 2013.

Icahn on Friday said Ackman had "developed a very bad case of Herbalife obsession."

While Ackman has lost money on his short bet over the years, Icahn has made money but his gains are now at risk of shrinking as the stock has fallen in the last few days.

The investors' comments come a little over a month after the company agreed to pay $200 million and change the way it does business to avoid being labeled a pyramid scheme by U.S. regulators.

Following the settlement with regulators, Herbalife said its board had cleared the way for Icahn to boost his stake in the company to as much as 35 percent.

Investment bank Jefferies Group has been trying to find buyers for Icahn's Herbalife stake for about a month, the Wall Street Journal reported on Friday.

Sources told Reuters on Thursday that Icahn was considering structuring a sale of Herbalife shares.

"We appreciate the support of all of our investors and are particularly grateful to Carl Icahn and the conviction he shares, and continues to show in our business," Herbalife CEO Michael Johnson said.

Ackman could not be reached for comment, while Jefferies was not immediately available for comment.

(Reporting by Bhanu Pratap, Sruthi Ramakrishnan and Gayathree Ganesan in Bengaluru; Additional reporting by Svea Herbst-Bayliss in Boston; Editing by Savio D'Souza, Saumyadeb Chakrabarty and Shounak Dasgupta)