WASHINGTON—President Trump said he is not planning to fire Federal Reserve Chairman Jerome Powell and he signaled that tariffs on China could be lowered, prompting relief from investors who had been spooked by the White House’s aggressive moves in recent weeks.

President Donald Trump on Monday kept up his verbal assault on Federal Reserve Chair Jerome Powell over the Fed's refusal for now to further cut interest rates, rattling financial markets growing ever more fearful that Trump may ultimately try to fire the U.