Rising tensions between China and the United States pummeled stocks again on Friday, as investors began to take seriously the risk of a trade war between the world’s two largest economies.

Markets began to slide at the start of trading after President Trump’s threat on Thursday to heap fresh tariffs on China, and the sell-off gained steam as the day progressed. A new report on hiring that showed the pace of job growth slowing in March — 103,000 jobs were added in the month — did little to buoy investors.

Investors took little comfort in statements from the administration’s top economic policymakers, Larry Kudlow, the director of the National Economic Council, and Steven Mnuchin, the Treasury secretary. Both men sought to tamp down the contentious back-and-forth between the two economic giants. On CNBC, Mr. Mnuchin emphasized that the United States was willing to negotiate with China, but he acknowledged that there was “potential for a trade war.”

And investors found few bright spots in afternoon comments from Jerome H. Powell, the new chairman of the Federal Reserve. In a speech in Chicago, Mr. Powell gave little indication that he planned to deviate from the Fed’s gradual path of lifting interest rates. He also suggested after his speech that he was unsure how the new tariffs imposed by the United States and China would affect the economy.