“We have no plans at this time to suspend any more exercises,” Mr. Mattis said.

Any resumption of large military exercises involving the Americans and South Koreans is certain to infuriate North Korean officials, who regularly denounce such operations. And the reversal reflects confusion within the Trump administration about how to deal with North Korea.

The president’s declaration of victory after his landmark summit meeting in Singapore in June has given way to tense accusations and counteraccusations. Mr. Trump has not yet returned to the verbal hostilities that he engaged in on his Twitter feed during his first year in office, when he referred to Mr. Kim as “Rocket Man” and raised the specter of bombing North Korea.

Part of the confusion, officials said, comes from the fact that Mr. Trump’s top deputies — particularly at the Pentagon — have not been consistently included in the president’s plans for North Korea. For instance, Mr. Trump’s decision in June to suspend the huge annual military exercises that had long been planned with South Korea took even senior American military officials by surprise.

Suspending the military exercises was widely seen as a concession to North Korea, and Defense Department officials said they expected the United States to receive something in return if the drills were shelved again.

The Trump administration is increasingly frustrated over the slow pace of negotiations and fears that North Korea is not making substantive moves toward dismantling and ending its nuclear weapons program — as Washington said was promised during the meeting in Singapore.