WASHINGTON  Fissures are developing among policy makers at the Federal Reserve as they debate how and when to start raising the benchmark interest rate from its current level just above zero.

With Fed officials forecasting that unemployment will average 9.8 percent in 2010, nobody appears to be arguing that monetary policy should be tightened anytime soon. The central bank’s official mantra continues to be that the overnight federal funds rate will remain “exceptionally low” for “an extended period.”

But Fed officials have hinted at new disagreement in recent weeks. The arguments go beyond the traditional split between hawks, who worry that easy money will stoke inflation, and doves, who contend that unemployment is the top problem.

The more devilish debates are about how fast to act once the decision has been made, and how to carry it out. Beyond raising the overnight federal funds rate, the Fed also has to unwind $2 trillion in special programs that prop up paralyzed banks and credit markets.