The Federal Reserve on Wednesday renewed its pledge to keep interest rates near zero for a "considerable time," but also indicated it could raise borrowing costs faster than expected when it starts moving.

Many economists and traders had expected the U.S. central bank to alter the rate guidance it has provided since March, given generally improving data on the economy's performance.

But the Fed repeated its assurance that rates would stay ultra-low for a "considerable time" after a bond-buying stimulus program ends. In a statement after a two-day meeting of its policy-setting Federal Open Market Committee, it announced a further $10 billion reduction in its monthly purchases, leaving the program on course to be shuttered next month.