WASHINGTON — Federal Reserve Chairman Ben Bernanke told Congress today that there’s increasing evidence that a “self-sustaining” economic recovery is taking hold, but he said the Fed’s $600 billion Treasury bond-buying program is still needed because it will take years for unemployment to drop to more normal levels.

In prepared testimony to the Senate Banking Committee, Bernanke offered a more optimistic outlook, saying the economy should grow more strongly this year as consumers and businesses boost their spending.

However, he said that even with the expected improvements, it could still take four to five years for unemployment to drop to a historically normal rate of about 6 percent.