The United States and Mexico agreed on Thursday to keep talking about a potential trade deal for 90 more days, averting the heavier tariffs President Trump had threatened to impose on America’s largest trading partner just before they were set to begin.

The news of the pause, announced by Mr. Trump and President Claudia Sheinbaum of Mexico on social media, came after the two leaders spoke by phone on Thursday morning. It also followed months of painstaking negotiations between the two sides that had left many Mexicans apprehensive about what Mr. Trump would ultimately decide.

The agreement will keep in place the 25 percent tariff on all goods from Mexico not covered by an existing free-trade agreement between the United States, Mexico and Canada. That rate had been set to rise to 30 percent on Friday, the deadline Mr. Trump set for countries across the globe to strike deals or face import taxes of as much as 50 percent.

“It was a very good agreement, and it puts our country in a very good situation,” Ms. Sheinbaum told reporters, adding that “mutual respect” was key to achieving the extension. “Within this new world trade order, we have the best possible agreement.”

In exchange, Mr. Trump said on social media, Mexico had agreed to “immediately terminate” its other trade barriers outside of tariffs, “of which there were many.” Mexican officials said they were discussing such matters, including labor-related mechanisms and disputes involving intellectual property.

But the announcement left many questions unresolved about the future trading relationship between United States and its two biggest trading partners, Mexico and Canada, especially as Mr. Trump has persistently married the threat of steep tariffs with political demands. In Mexico’s case, he has pushed hard for Ms. Sheinbaum to do more to battle drug cartels, curb the flow of fentanyl and stop migration toward the United States.