WASHINGTON U.S. Treasury Secretary Janet Yellen said on Tuesday she was very optimistic about the outlook for the U.S. economy.

Yellen also said interest rates would return to more normal levels in the medium term.

Commenting on artificial intelligence, Yellen said progress in that field was "unbelievably rapid" and could make a significant difference in productivity.

(Reporting by David Lawder and Kanishka Singh in Washington; Editing by Jonathan Oatis and Gareth Jones)