Federal Reserve Chairman Jerome Powell repeated his pledge Friday to keep the economic expansion going while acknowledging that tariffs and other factors are causing growth to slow. Less than an hour after the speech, President Donald Trump blasted Powell on Twitter, referring to him as "our enemy."

Powell, while not saying specifically where he thought rates should go, promised that the Fed "will act as appropriate to sustain the expansion," a phrase he has used several times in the recent past.

Powell also said in his annual remarks at the central bank's Jackson Hole symposium that the "economy is close to both goals" of the Fed's dual mandate of full employment and price stability.

"Our challenge now is to do what monetary policy can do to sustain the expansion so that the benefits of the strong jobs market extend to more of those still left behind, and so that inflation is centered firmly around 2 percent."

He also outlined the challenges the Fed faces and indicated that for he and his fellow officials there are "no recent precedents to guide any policy response to the current situation."

"While monetary policy is a powerful tool that works to support consumer spending, business investment, and public confidence, it cannot provide a settled rulebook for international trade," he said in prepared remarks. "We can, however, try to look through what may be passing events, focus on how trade developments are affecting the outlook, and adjust policy to promote our objectives."

He did say the Fed is looking at ways to address developments in a landscape that has changed significantly since the expansion began a decade ago.

"We are examining the monetary policy tools we have used both in calm times and in crisis, and we are asking whether we should expand our toolkit," he said.