Federal Reserve Chairman Jerome Powell said Wednesday the U.S. economy could face headwinds in the next 18 months to two years from several factors.

Powell cited slowing global growth; the waning effect of tax cuts and federal spending increases; and the potential delayed impact of rate hikes imposed by the Fed in recent months.

The Fed chairman noted that the U.S. economy itself is performing well. "I'm very happy about the state of the economy," he said, noting low unemployment rates and growth around 3%. "Our goals will be to extend the recovery, keep unemployment low and keep inflation low," he added.

Powell was interviewed by Dallas Federal Reserve Bank President Robert Kaplan in front of an audience at the bank's Texas headquarters.

Powell noted that while 2017 was generally a good year for global growth, "This year has seen a gradual chipping away." He noted that "far more than half of the growth in the world comes from emerging market countries" including China and India.

He said the effects of President Trump's tariffs on Chinese goods imported to the U.S. have not been seen clearly in economic data, but that anecdotal evidence from business leaders has been increasing.

Powell said global trade has been beneficial to the U.S. economy, adding "If this process that's going on right now winds up with lower tariffs and better observance of agreed trade rules, then that will be good for our economy and good for the world economy. If it instead winds up in a place where we have more widespread protectionism around the world that will be bad for our economy."

Read more on Real Money: Jim Cramer: We Need to Think About the Outcomes With the Fed, China.