Federal Reserve Bank of San Francisco President John Williams reiterated Wednesday that the Fed could start to raise its interest rate this year as the U.S. economy continued to expand.

"I still believe this will be the year for lift-off, and I still believe that waiting too long to raise rates poses its own risks," he said.

But he added that he cannot tell the date.

San Francisco Fed president said that he does not expect the situation in China and Greece will have a negative impact on the U.S. economy.

Williams pointed out that lower oil prices had less of a positive impact on the U.S. economy, and the reasons are unclear.

Williams is a voting member of the Federal Open Market Committee this year.