The $700bn US bail-out package is "not a panacea" to cure economic woes, says Treasury Secretary Henry Paulson, reported BBC.
Mr Paulson and Federal Reserve chairman Ben Bernanke have been called to give evidence about the Troubled Asset Relief Program (TARP) scheme.
It was authorised by Congress last month to shore up the financial system.
"It will take a while to get lending going and repair our financial system, which is essential to an economic recovery," Mr Paulson said.
He also told Congress it was vital the administration be nimble in assessing changing conditions and adapting the bail-out strategy accordingly.
"If we have learned anything throughout this year, we have learned that this financial crisis is unpredictable and difficult to counteract," Mr Paulson said.
Instead of buying up the banks' toxic mortgage debts, as first proposed by the rescue deal, the bail-out fund will continue to be used to buy shares in the lenders to help boost their balance sheets.
Democrats want the bail-out funds to be used to help the struggling US auto industry, while Mr Paulson says it is also important to help other financial institutions in difficulty, including credit card companies and other non-banks who lend to consumers.