Federal Reserve Chairman Ben Bernanke says it's a challenge for the government to stay ahead of counterfeiters as technology becomes more sophisticated and more dollars flow overseas.

Bernanke's brief remarks come at the unveiling of a redesigned $100 bill. The government hopes the new bills will make it harder for high-tech counterfeiters to knockoff.

The Fed chief says: Staying ahead of bogus bills is a job that has become "more complex in recent years as technology advances and U.S. dollar flows expand and increase." He says as many as two-thirds of all $100 bills circulate outside the United States.

The new bills goes into circulation on Feb. 10, 2011.