President Trump suggested on Friday that the trade truce between the United States and China was not holding and accused Beijing of breaking an agreement that was brokered this month to temporarily roll back trade restrictions that the countries had imposed on each other.

In a post on Truth Social, Mr. Trump said that China violated the pact and suggested that he could return to a more confrontational approach: “So much for being Mr. NICE GUY!”

The accusations threatened to derail hopes of a broader trade agreement between the world’s largest economies. China has continued to restrict exports of critical rare earth minerals, straining ties between the countries. In response, the United States this week ratcheted up its own restrictions of critical components. And each country has targeted imports from the other with round after round of tariffs in recent months.

The standoff between the United States and China has created significant concern for businesses and investors and raised fears of a global downturn.

Stocks fell slightly on Friday in the wake of Mr. Trump’s post.

The new dispute arrives at a moment of great uncertainty for Mr. Trump and his ability to brandish steep tariffs to force other countries to make trade concessions. A federal trade court earlier this week declared many of the president’s duties to be illegal, including some that he imposed on China on emergency grounds. An appeals court later restored that power temporarily.

The United States had ratcheted tariffs on Chinese imports to 145 percent earlier this year, and China had hit American products with a 125 percent import tax. Beijing also moved to set up a licensing system for exports of rare earths, which are needed by U.S. manufacturers of cars, semiconductors and airplanes, including the defense industry. Those restrictions threatened to cripple vital American industries, which lobbied the Trump administration for relief.