The U.S. Federal Reserve's exit from its ultra-low interest rate policy will take a significant period of time, a San Francisco Fed economist said in a paper published on Monday.

Downplaying the inflationary implications of the Fed's unconventional easing measures, Glenn Rudebusch, the regional central bank's associate director of research, indicated the Fed is not inclined to tighten policy any time soon.

Many predict that the economy will take years to return to full employment and that inflation will remain very low, Rudebusch wrote. If so, it seems likely that the Fed's exit from the current accommodative stance of monetary policy will take a significant period of time.

That view reflects the thinking of San Francisco Fed President Janet Yellen, widely seen as one of the biggest monetary doves on the U.S. central bank's policy-setting Federal Open Market Committee.

Her approach to policy, which is closely aligned with that of Chairman Ben Bernanke, might soon become even more influential, since President Barack Obama recently nominated Yellen to take over from Donald Kohn as Fed Vice Chairman. Her appointment is pending Senate approval.

In response to the most severe financial crisis in generations, the Fed not only chopped interest rates down to almost zero but also undertook a wide array of emergency steps, including large scale purchases of long-term Treasury and mortgage bonds.

In the process, outstanding Fed credit to the banking system, known in the markets as the central bank's balance sheet, expanded sharply to around $2.3 trillion.

Yet Rudebusch argues that the large sums have few implications for inflation for as long as banks are reluctant to lend and demand for credit is scant.

The doubling of the Fed's balance sheet has had no discernible effect on long-run inflation expectations, he said.

Given predictions for the jobless rate to come down only slowly and inflation to remain low, there is little need to raise the (federal) funds rate target above its zero lower bound any time soon, Rudebusch says.