The U.S. Federal Reserve Bank may get little bang for its buck if it tries to help boost the modest economic recovery with a renewed round of Treasury debt purchases, Minneapolis Fed President Narayana Kocherlakota said on Thursday.

But Kocherlakota, who has been cool to the idea of further Fed monetary policy easing because he attributes at least some of the high U.S. unemployment rate to factors beyond the Fed's control, stopped short of joining some of his colleagues, including Kansas City Fed President Hoenig, in firm opposition.

Instead, he repeated his view that weighing its costs and benefits is a difficult "calculus."

The Fed has kept benchmark U.S. interest rates near zero since December 2008, and bought $1.7 trillion in mortgage-backed securities and Treasuries to pull the U.S. economy out of its worst recession in decades.

But U.S. growth slowed over the summer, and nearly one in 10 in the labor force are unable to find work.