NEW YORK (Reuters) – Hedge-fund and private-equity firm Fortress Investment Group (FIG.N: Quote, Profile, Research, Stock Buzz) reported a sharp rise in fourth-quarter results, boosted by strong fee revenue from its credit funds and its Logan Circle acquisition.

But the company’s stock price fell 6.9 percent on concern its recent rapid rise may not be sustainable, analysts said. Shares in Fortress, which had been one of the hardest-hit firms in the financial crisis, ran up sharply ahead of the earnings report and are up 34 percent since the start of December.

Even though the firm beat analyst estimates in the fourth quarter, it may have “limited upside potential,” Standard & Poor’s analyst Rafay Khalid said in a note to clients on Tuesday.

Fortress, which was one of the world’s first big hedge-fund and private-equity firms to go public, said it generated incentive income for its portfolios in all its segments — with sharp rises in its credit hedge funds, liquid hedge funds, credit private equity funds and private equity funds — for the first time since before the financial crisis.

Fortress said it raised $5.3 billion from 233 investors in 2010, tripling the amount it had raised from just 93 investors in 2009. The firm had $44.6 billion in assets under management as of December 31.

Chief executive Dan Mudd said he expects Fortress to continue its capital-raising push in the next few months.

“As corporations restructure and recapitalize, and refocus and prepare for whatever the new normal is, we expect opportunities to be robust for some period of time,” Mudd added on a conference call with investors.