Last week's vote in the U.K. to leave the European Union has heightened risks to an already fragile global economy and prompted the Federal Reserve to adopt a more patient posture as it considers future interest-rate moves, Fed governor Jerome Powell said.

In the aftermath of the vote, "global risks have now shifted ever further to the downside," Mr. Powell said in remarks prepared for delivery at an event in Chicago Tuesday. "The Brexit vote has the potential to create new headwinds for economies around the world, including our own."

Mr. Powell did not explicitly say how the central bank would respond to the stunning vote, but suggested officials were in no hurry to raise interest rates again.

"Monetary policy will remain supportive of growth, as we work through the challenging global environment," he said. The Fed is "closely monitoring developments" and is ready to help other central banks through its swap lines, he said.

Mr. Powell's is the first speech from a Fed official since the vote.

The Brexit outcome shocked markets and central bankers around the world, many of whom had not expected U.K. voters to reject the European Union so soundly. Markets plunged in the days following the vote before recovering somewhat on Tuesday. Rapid currency swings sent the dollar and the yen soaring while the British pound fell, leading European Central Bank President Mario Draghi to implore monetary policy officials to resist the temptation to devalue their currencies to boost exports.

Federal Reserve officials said earlier this month they expect to raise interest rates by a half percentage point this year, likely in two increments. But the Brexit vote may have scrambled those plans.

"As the global outlook evolves, it will be important to assess the implications for the U.S. economy, and for the stance of policy appropriate to foster continued progress," Mr. Powell said.

The Fed raised short-term interest rates to between 0.25% and 0.5% in December.

The Fed governor noted, however, that the financial system had performed well under pressure.

"Although financial conditions have tightened since the vote, markets have been functioning in an orderly manner," he said. "And the U.S. financial sector is strong and resilient."

Mr. Powell said the Fed had kept interest rates low for so long because the neutral real interest rate, which is the level that would bring about full employment while holding inflation steady, had dropped substantially during the crisis. Although the neutral rate is impossible to observe, Mr. Powell said, he thought it had fallen to about zero.

Since the federal-funds rate, after taking inflation into account, now stands at about -1.25%, "policy is actually only moderately stimulative," he said.

Over time, he said, the labor market would continue to improve and inflation would rise to the Fed's 2% target, which would lift the neutral rate. In the long term, however, Mr. Powell said he was worried that a decline in productivity had reduced the economy's potential growth rate.

Write to David Harrison at david.harrison@wsj.com

(END) Dow Jones Newswires

June 28, 2016 20:35 ET (00:35 GMT)