WASHINGTON — Janet Yellen, the Federal Reserve chairwoman, said Wednesday that stock prices were “quite high,” stronger language than she had used previously.

Ms. Yellen reiterated the Fed’s view that risks to financial stability remained modest, despite years of near-zero interest rates, in part because there had not been a marked increase in the use of borrowed money, or leverage, to fund investments.

But she cautioned that the volatility of financial markets could increase when the Fed begins to raise its benchmark interest rate. She noted that the Fed has held that rate near zero for “an unthinkable six plus years,” beginning in December 2008.

Ms. Yellen’s remarks, made at a policy conference where she appeared with Christine Lagarde, the managing director of the International Monetary Fund, sounded a more wary note than her assessment of financial market conditions earlier this year. In February, Ms. Yellen described stock prices as “somewhat higher than their historical average levels.” In March, she described valuations as “on the high side.”