WASHINGTON — Defense Secretary Jim Mattis opened the door on Tuesday to restarting large-scale military exercises on the Korean Peninsula, appearing to contradict President Trump, who had labeled the war games costly and “provocative” two months ago in trying to lower tensions with North Korea.
Mr. Mattis insisted that the step did not signal that the era of détente between the Trump administration and Kim Jong-un, the North Korean leader, was over.
But the Pentagon chief’s comments came amid souring relations: In the last week alone, the United States canceled a trip by Secretary of State Mike Pompeo to Pyongyang and North Korean state media accused the United States of “double-dealing attitudes” and “extremely provocative and dangerous military moves.”
“We took the step to suspend several of the largest exercises as a good-faith measure coming out of the Singapore summit,” Mr. Mattis told reporters at the Pentagon on Tuesday, referring to Mr. Trump’s decision to shelve large-scale drills with South Korea after meeting with Mr. Kim in June.
“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.
In fact, North Korea never agreed to a process to get rid of its nuclear weapons; experts estimate that it has as many as 60 warheads. It also has been developing a ballistic missile that can deliver a warhead to the mainland United States.
At a talk on Tuesday at the Foundation for Defense of Democracies in Washington, Nikki R. Haley, the United States ambassador to the United Nations, said it was “possible” that North Korea was changing its mind on a commitment to denuclearization.
During a rare Pentagon news conference on Tuesday, Mr. Mattis sidestepped a question about whether he believed North Korea’s nuclear threat had abated, as Mr. Trump has claimed. Mr. Mattis said the “whole world” saw “progress when the two leaders sat down” in Singapore.
But, he added, “we also knew very clearly this was going to be a long and challenging effort to negotiate this away.”
At the State Department, Heather Nauert, the chief spokeswoman, said Mr. Pompeo remained committed to finding a diplomatic solution — but only if Mr. Kim takes sufficient steps “to completely denuclearize North Korea.”
“We stand ready to talk when the North Koreans stand ready to do so,” she said.
One American official familiar with the plans said that the president canceled Mr. Pompeo’s trip to Pyongyang after the top diplomat received a letter — which news reports have described as belligerent — from a senior North Korean official.
The letter was delivered last week, just hours after Mr. Pompeo announced both the trip and a new senior envoy for North Korea. The official spoke on condition of anonymity, and Ms. Nauert declined to confirm the letter or provide details on what she described as “private diplomatic conversations.”
She said John R. Bolton, the national security adviser, was also consulted on the decision to cancel the trip to Pyongyang; he had called in from abroad on a secure line during a White House meeting with Mr. Trump and Mr. Pompeo.
“The president remains very cleareyed about the challenges and difficulties ahead of us,” Ms. Nauert said.
She cited the return of the remains of American troops who had died in the Korean War and the release in May of three American prisoners as diplomatic progress. But, she said, Mr. Trump and other officials had deemed that those and other steps by Pyongyang were not sufficient to justify a new trip by Mr. Pompeo.
Bruce Klingner, a Korea analyst at the Heritage Foundation and former C.I.A. officer, said he suspected that Pyongyang had reiterated its longstanding position that its understanding of denuclearization was that it would give up its nuclear arms when other world powers did the same.
“The North Koreans have been very clearly articulating their longstanding positions,” Mr. Klingner said. “Some say it is negotiating tactics. But look at what Kim Jong-un has said: ‘We will never abandon the sword of nuclear arms.’”
If the Pentagon restarts large-scale training exercises on the Korean Peninsula, the first one would most likely be an annual drill usually held in the spring and known, alternately, as Foal Eagle or Key Resolve.
It was held this year after the Winter Olympics in Seoul, and involved around 23,000 American and 300,000 South Korean troops. The exercise involves military units training on the sea, air and land.
However, within minutes of declaring on Tuesday that the drills could resume, Mr. Mattis added caveats to his comments. He said no decision has been made about whether the annual exercises, including the Ulchi Freedom Guardian exercises that Mr. Trump had ordered canceled, would be held next year.
He declined to comment on whether restarting large-scale military exercises could be viewed as provocative.
“I don’t have a crystal ball,” he said. “Even answering a question in that manner could influence the diplomatic effort.”
Julian E. Barnes contributed reporting.