Janet Yellen, the U.S. Federal Reserve's influential vice chair, said on Monday the central bank's aggressive monetary stimulus is warranted given how far below its full potential the economy is operating.

Downplaying the potential costs of the Fed's unconventional easing efforts, which currently include $85 billion in monthly asset purchases, Yellen highlighted the dangers of a prolonged period of economic malaise.

"Insufficiently forceful action to achieve our dual mandate also entails costs and risks," Yellen told a conference sponsored by the National Association of Business Economists. "At present, I view the balance of risks still calling for highly accommodative monetary policy to support a stronger recovery and more rapid growth in employment."

(Reporting by Pedro Nicolaci da Costa; Editing by Chizu Nomiyama)