"I expect that inflation will move back down ... on a monthly basis to levels that were more like the lower readings that we were having last year," Powell said at a banking event in Amsterdam. "I would say my confidence in that is not as high as it was."

Still, Powell said he felt it unlikely that the Fed would have to raise interest rates again, restating as he did after the Fed's last meeting that the central bank will be "patient" and allow the current policy rate to have its full impact.

"I don't think that it is likely based on the data we have that the next move that we make will be a rate hike," Powell said. "It is more likely ... we hold the policy rate where it is."

"We did not expect this to be a smooth road," Powell said.

Powell spoke shortly after new data showed producer prices in April rose more quickly than expected, a possible sign of pressure building on the prices charged to consumers.

But Powell said he took the data as "quite mixed," noting that input prices for prior months were revised lower.

The Fed has kept its benchmark policy rate steady in a 5.25% to 5.5% range since July.