STONE MOUNTAIN, Ga.—Federal Reserve Chairwoman Janet Yellen said Tuesday that the largest and most complex financial institutions may be able to strengthen their balance sheets further by raising more capital than regulators have required to date.

"There might be room for stronger capital and liquidity standards for large banks than have been adopted so far," Ms. Yellen said in a video recording played at a financial markets conference sponsored by the Federal Reserve Bank of Atlanta.

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