Citigroup Inc shares fell to 17-year lows on Thursday, with Bank of America Corp stock also plunging, amid renewed fears that growing losses could lead to government control of troubled U.S. banks, wiping out shareholders.

Bank of America shares were down 10.7 percent to $4.07 in morning trading on the New York Stock Exchange, while Citigroup stock fell 10 percent to $2.62, after touching its lowest level since 1991.

When you talk about nationalization you hear the names Citi and Bank of America as the top two names burning out, said Walter Todd, a portfolio manager at Greenwood Capital Associates.

We still don't have any clarity about what course of action they (the government) are going to take. In the absence of specifics, people just jump to their own conclusions, he added.

Last month, Bank of America Corp posted its first quarterly loss in 17 years, after mounting losses at recently acquired Merrill Lynch. Citigroup has lost $28.5 billion in the last 15 months, hammered by bad debts and toxic assets.

Each bank received $45 billion in government aid in recent months and a backstop on toxic assets-related losses.

(Reporting by Juan Lagorio, editing by Matthew Lewis)