The stock market slumped to its worst one-day showing since tariffs roiled markets in April, as the specter of the trade war returned between Beijing and Washington.

The S&P 500 dipped more than 2 percent for the first time in six months, and the Nasdaq Composite also fell by the most it has since April, when President Trump’s tariff plans for dozens of countries spooked markets, prompting the administration to quickly rein in its proposals.

Mr. Trump on Friday threatened to impose more tariffs on Chinese imports after its government put curbs on the export of rare earth materials, vital to a host of industries, including the production of valuable chips used in artificial intelligence.

“One of the Policies that we are calculating at this moment is a massive increase of Tariffs on Chinese products coming into the United States of America,” Mr. Trump wrote on his social media site TruthSocial. “There are many other countermeasures that are, likewise, under serious consideration.”

Friday’s decline was the first time in two months that the S&P 500 had fallen more than one percent.

The stock market was already on edge this month, after five straight months of gains had lifted prices to record levels, prompting some investors to warn of the potential for a pullback. The drop on Friday remained fairly modest, with the S&P 500 still less than 3 percent from its record high.

“Is this the start of a Liberation Day Two?” asked Andrew Brenner, the head of international fixed income at National Alliance Securities, in an emailed note to clients on Friday afternoon.