Federal Reserve Chairman Ben Bernanke says some of the problems that are slowing the U.S. economy could persist into next year.

Bernanke said at a news conference Wednesday that the slowdown could be due, in part, to the depressed housing market and other factors that aren't likely to fade soon.

"Maybe some of the headwinds that are concerning us, like the weakness in the financial sector, problems in the housing sector _ some may be stronger and more persistent than we thought," he said.

Bernanke described the debt crisis in Greece as a "very difficult situation." He said that if Greece defaulted on its debt, the impact would go beyond Europe and threaten the global economy.

In answer to another question, Bernanke said the impact on financial institutions would likely be "very small." But he said a spiraling Greek debt crisis that roiled financial markets would pose more severe threats.

The Fed chairman made the comments at his second news conference of the year. Under a new Fed policy, he plans to take questions from reporters four times each year.

Bernanke was asked about the potential timetable for the Fed to keep its main interest rate near zero for "an extended period" to stimulate the economy. He said the continued use of "extended period" in the Fed's statements meant it is at least two or three meetings away from raising rates. The Fed meets every six to eight weeks.

Most private economists say they think the Fed won't begin raising rates for another full year.