WASHINGTON — North Korea threw President Trump’s planned summit meeting with its leader, Kim Jong-un, into doubt on Tuesday, threatening to call off the landmark encounter to protest a joint military exercise of the United States and South Korea.
The warning, delivered early Wednesday in North Korea via its official government news agency, caught Trump administration officials off guard and set off an internal debate over whether Mr. Kim was merely posturing in advance of the meeting in Singapore next month or was erecting a serious new hurdle.
North Korea had previously signaled flexibility about the military exercises, appearing to remove a perennial obstacle to talks between North and South Korea. But the North also cited its objections to the joint American-South Korean air force drill in postponing a separate high-level meeting with South Korea that had been planned for Wednesday.
As the White House scrambled to assess the North Korean statement, the State Department said planning for the June 12 summit meeting remained on track, and pointed to Mr. Kim’s earlier acceptance of the exercises, which had been conveyed to the United States by South Korean officials.
The North Korean statement injected sudden tension and uncertainty into what had been months of warming relations on the Korean Peninsula, most notably the summit meeting between Mr. Kim and President Moon Jae-in of South Korea on April 27 and their declaration to pursue peace.
The confusion created by the statement also underscored the risks for Mr. Trump in meeting with the 34-year-old North Korean leader. And it served as a reminder that for all of Mr. Kim’s camera-ready smiles and diplomatic gestures of recent months, North Korea remains an opaque, unpredictable country.
“The South Korean authorities and the United States launched a large-scale joint air force drill against our Republic even before the ink on the historic inter-Korean declaration has dried,” the official Korean Central News Agency said. “There is a limit to our good will.”
“We will be closely watching the attitude of the United States and South Korean authorities,” the news agency added.
It declared that the drill, known as Max Thunder, was a “deliberate military provocation” that had violated the inter-Korean summit declaration. The United States and South Korea, the North’s statement said, had mobilized 100 aircraft in the exercise to “make a pre-emptive airstrike” and “win the air.”
Senior officials from the two Koreas had been scheduled to meet in the “truce village” of Panmunjom on their border Wednesday to discuss putting in place an agreement to improve ties and ease military tensions, building on the declaration signed by their leaders on April 27.
The Pentagon said Max Thunder was an annual exercise to maintain military readiness to defend South Korea. “The defensive nature of these combined exercises has been clear for many decades and has not changed,” said a Defense Department spokesman, Col. Rob Manning.
The State Department spokeswoman, Heather Nauert, said: “Kim Jong-un had said previously that he understands the need and the utility of the United States and the Republic of Korea continuing in its joint exercises. They are exercises that are legal. They’re planned well, well in advance.”
She also said the United States had received no notification of a change in plans for the summit meeting. “We will continue to go ahead and plan the meeting between President Trump and Kim Jong-un,” she said.
There was some ambiguity about whether Mr. Kim had dropped his objection to all joint exercises, or merely the ones that were underway at the time that he opened a dialogue with Mr. Moon. When Mr. Kim met with South Korean envoys in March, he agreed to meet with Mr. Moon despite a round of joint military drills that were about to be conducted.
South Korean envoys quoted Mr. Kim as saying he understood that those exercises must continue. But those drills are over. The new round of annual United States-South Korean drills began last Friday.
In their March meeting, Mr. Kim told the South Korean emissaries that he hoped the United States and South Korea would “readjust” their annual military drills “when the situation stabilizes,” according to the envoys.
Several experts said North Korea’s warning was probably a bump in the road rather than a dire threat to next month’s meeting. But they said it pointed up the complexity of the negotiations that Mr. Trump faces.
“It lays down a probable marker that exercises will be on the table in negotiations,” said Victor D. Cha, a Korea scholar at Georgetown University whom the Trump administration had considered as ambassador to Seoul.
Joel S. Wit, a senior fellow at the Johns Hopkins School of Advanced International Studies who negotiated with North Korea during the Clinton administration, said Mr. Kim was pulling a page from Mr. Trump’s playbook.
“It’s probably them acting like North Koreans after being pussy cats since January,” he said. “Acting like tough guys. Like Trump saying he would walk out of the summit if he didn’t like the deal.”
Christopher R. Hill, who negotiated with North Korea during the George W. Bush administration, said it was possible that the threat was more serious. The North, he noted, has a history of insulting the South, and normally, the United States comes to the defense of its ally.
Mr. Trump did not immediately react to Mr. Kim’s warning, and the White House issued only a bland statement. “We are aware of the South Korean media report,” the press secretary, Sarah Huckabee Sanders, said. “The United States will look at what North Korea has said independently, and continue to coordinate closely with our allies.”
While Mr. Trump has raised expectations for a breakthrough with Mr. Kim, other officials have tried to strike a more cautious note. In an interview on the CBS program “Face the Nation” last Sunday, Secretary of State Mike Pompeo, who has met Mr. Kim twice, said, “We have our eyes wide open with respect to the fact that the North Koreans have not proved worthy of their promises, but we’re hopeful that this will be different.”
Tensions on the divided Korean Peninsula have eased considerably since Mr. Kim launched a fast-paced string of diplomatic overtures in recent months, starting with his decision to send North Korean athletes to the Winter Olympics held in the South in February.
Last month, North Korea announced an end to all nuclear and long-range missile tests. Last week, it freed three Americans held in North Korea, sending them home with Mr. Pompeo.
This week, the North invited international journalists to watch its engineers shut down its only-known nuclear test site later this month. The two Koreas have also started dismantling loudspeakers they have used to blare propaganda broadcasts across the border.
It is not unusual for North Korea to abruptly cancel and postpone meetings with its neighbors. The North’s decision to postpone the border talks was delivered only 15 hours after it proposed those talks on Tuesday and the South quickly accepted the offer.
In 2015, a North Korean band packed up and returned home only hours before it was scheduled to perform in Beijing in a gesture of friendship between the two countries’ Communist governments. In January, North Korea agreed to send an advance team of officials to the South to prepare for its participation in the Winter Olympics, but abruptly delayed their trip.
“Seoul is a soft target,” said Lee Sung-yoon, a Korea expert at the Fletcher School of Law and Diplomacy at Tufts University, referring to the North’s history of “periodically threatening to walk away from talks” in order to raise its leverage during negotiations. “The North will resume the canceled talks in time — likely as early as later in the week.”