The risks of asset-price bubbles are growing now that the U.S. economy is improving, St. Louis Federal Reserve President James Bullard said on Wednesday, adding he would not be opposed to using monetary policy to head off any risks that might develop.

"As the economy improves ... and we continue to have low rates, that's a fertile environment for creation of asset bubbles in the future," Bullard told reporters at the New York Stock Exchange. "So I would start to put more weight now on that as a risk than I would have last year or the year before."