Federal Reserve Chair Janet Yellen sees a number of encouraging signs that the economy is reviving after a brutal winter and says if the improvements continue, the Fed will likely start raising interest rates later this year.

Delivering the Fed's mid-year economic outlook to Congress, Yellen says the importance of the first rate hike should not be over-emphasized because interest rates are likely to remain at very low levels "for quite some time after the first increase."

The Fed's benchmark rate has been at a record low near zero since December 2008, meaning that borrowing rates for consumers and businesses have been at historic lows.

Many economists believe the Fed's first rate hike will occur in September. But they see at most only two quarter-point moves this year.