The Federal Reserve on Friday left the door open to a September interest rate hike even while several U.S. central bank officials acknowledged that turmoil in financial markets, if prolonged, could delay the first policy tightening in nearly a decade. Some top policymakers, including Fed Vice Chairman Stanley Fischer, said recent volatility in global markets could quickly ease and possibly pave the way for the U.S. rate hike, for which investors, governments and central banks around the world are bracing. With a key policy meeting set for Sept.