WASHINGTON — Federal Reserve Chairman Ben Bernanke said Wednesday he did not know when the Fed would tighten interest rates but that the “extended period” described in the central bank’s statements suggests it will wait at least a “couple of meetings” before acting.

Bernanke also said during an unprecedented press conference that the US dollar’s weakness in part reflects an unwinding of the safe-haven flows the greenback saw during the financial crisis.

He noted separately that the US economy likely grew at a rate below 2 percent in the first quarter.

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