"I think that systemic risks are still out there," Obama said in an interview on the CBS program "60 Minutes."
"If we did nothing you could still have some big problems. There are certain institutions that are so big that if they fail, they bring a lot of other financial institutions down with them."
Obama said when he took office that he had not expected to see such a sharp economic decline, particularly in job losses.
"I mean if you look at just ... hundreds of thousands, now millions of jobs being shed over the course of two months, or three months -- that slope is a lot steeper than anything we've ... seen before," he said.
But he added there was a "potential silver lining" -- that the recovery might be equally as fast as the downturn.
Asked when he thought the economic crisis might end, Obama said there were already "flickers of hope" with low interest rates and an increasing number of mortgage refinancings.
"That promises the possibility at least of the housing market bottoming out and stabilizing," he said. "It's not going to happen equally in every part of the country.
Obama said there was a limit to the amount of money that the U.S. government could spend or print to help improve the economy.
"The limit is our ability to finance these expenditures through borrowing," he said. "The dollar is still strong because people are still buying Treasury Bills. They still think that's the safest investment out there."
"If we don't get a handle on this, and also start looking at our long-term deficit projections, at a certain point people will stop buying those Treasury Bills."
Obama also stepped up his weeklong defense of Treasury Secretary Tim Geithner and said he would not accept his resignation even if it was tendered.
He said if Geithner tried to quit, Obama said he would tell him, "Sorry buddy, you've still got the job."
0:00/2:31Geithner's fix for undue bonusesGeithner has been under fire over his failure to block at least $165 million and possibly as much as $218 million, in bonuses paid out to employees of insurer American International Group, which has received billions in government aid.
In the interview, Obama said he was waiting to look at the legality of legislation that would impose a tax of up to 90 percent on bonuses awarded by AIG or by some companies that have received government bailouts.
On the subject of the automobile bailout, Obama acknowledged that "the only thing less popular than putting money into banks is putting money into the auto industry."
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