Federal Reserve officials planning to lift interest rates as soon as September have been encouraged by solid U.S. jobs growth, but inflation holds the key to how far the Fed can go in moving rates away from zero. Fed officials have said that they do not need to see prices accelerate to start raising rates after six years near zero, and “lift-off” appears nearly ordained by a 5.3 percent unemployment rate, the lowest since April of 2008. If prices remain stalled as the Fed tightens, inflation-adjusted “real” rates would rise faster than the Fed wants, and threaten the recovery.