Federal Reserve Chair Janet Yellen said that additional gradual interest-rate increases will be appropriate but was silent on their precise timing, an omission viewed as a signal that a June move was off the table.

"I continue to think that the federal funds rate will probably need to rise gradually over time to ensure price stability and maximum sustainable employment in the longer run," Yellen said Monday during a speech in Philadelphia.

Her comments were less specific than in her previous remarks in describing when she thought the Fed should raise rates again. On May 27 at Harvard University, she said an increase would likely be appropriate in "coming months," a phrase she didn't repeat on Monday. Since then, the Labor Department reported U.S. employers in May added the fewest number of new jobs in almost six years, causing expectations for a rate increase to plunge.