"Economic activity decelerated somewhat over the first half of this year," the Federal Reserve announced after a two-day top-level meeting on Wednesday.
The interest rate-setting Federal Open Market Committee (FOMC) also said that it expected "economic growth to remain moderate over coming quarters and then to pick up very gradually."
"The unemployment rate will decline only slowly," it added.
Despite the disappointing news, no measures have been taken by government officials so far, and the Federal Reserve shied away from launching a fresh economic stimulus package.
Ryan Sweet of Moody's Analytics described the Fed's decision not to provide more stimulus or extend the timeframe for low rates as "a bit of (a) surprise move."
"There was a strong case for changing the rate guidance," he said, adding that "the odds of the Fed launching a third round of quantitative easing in September are lower."