The Federal Reserve's unconventional monetary stimulus has its limits, and could pose threats to market functioning and financial stability if pushed too far, Atlanta Fed Bank President Dennis Lockhart said.

In another sign of growing reticence about the Fed's bond-buying quantitative easing program within the central bank, Lockhart, seen as a policy centrist who tends to fall in line with Chairman Ben Bernanke, said the open-ended approach to bond buys does not mean there are no constraints on the policy.