Federal Reserve Chair Janet Yellen is signaling her belief that the U.S. economy is improving but remains defined by so many uncertainties that it's unclear when the Fed should resume raising interest rates.

Speaking in Philadelphia, Yellen dropped a reference she had made on May 27 that a rate hike would likely be appropriate "in the coming months." Instead, she acknowledged that a dismal jobs report that the government issued Friday had raised some doubts about the strength of the economy.

Yellen's speech had been highly anticipated given that it comes just a week before the Fed's next meeting on June 14-15.