WASHINGTON — Secretary of State Rex Tillerson let slip last week a few tantalising details about one of the nation’s most secret military contingency plans: how the United States would try to race inside North Korea to seize its nuclear weapons if it ever saw evidence that Kim Jong Un’s government was collapsing.

For years, US diplomats have been trying to engage their Chinese counterparts in a discussion of this scenario, hoping to avoid a conflict between arriving US Special Forces — who have been practicing this operation for years — and the Chinese military, which would almost certainly pour over the border in a parallel effort.

And for years the Chinese have resisted the conversation, according to several former US officials who tried to engage them in joint planning. The Chinese feared that if news of a conversation leaked, Beijing would be seen as conspiring with the United States over plans for an eventual North Korean collapse, eroding any leverage that Beijing still held over Kim.

So it was surprising to Tillerson’s colleagues in the White House and the Pentagon when, in a talk to the Atlantic Council last week, he revealed that the Trump administration had already provided assurances to China’s leadership that if US forces landed in North Korea to search for and deactivate nuclear weapons, the troops would do their work and then retreat.

North Korea has defied past predictions of collapse, and one does not appear imminent. But if a collapse were to occur, the aftermath could present grave dangers. US officials have envisioned that North Korean officers, fearing the end of Kim’s government, might lob a nuclear weapon at South Korea or Japan as a last, desperate act — or detonate it on North Korean territory to make occupation impossible.

On Tuesday, speaking from note cards, Tillerson said at a conference on the Korea crisis that the United States and China “have had conversations about in the event that something happened — it could happen internal to North Korea; it might be nothing that we from the outside initiate — that if that unleashed some kind of instability, the most important thing to us would be securing those nuclear weapons they’ve already developed and ensuring that they — that nothing falls into the hands of people we would not want to have it.”

He added, “We’ve had conversations with the Chinese about how might that be done.”

He repeated his past assurance that the administration was not seeking “regime collapse” or “an accelerated unification of the Korean Peninsula.”

“We do not seek a reason to send our own military forces north of the Demilitarized Zone,” the dividing line between North and South, he said.

But if the United States’ hand is forced, he added, “we have had conversations that if something happened and we had to go across a line, we have given the Chinese assurances we would go back and retreat back to the south of the 38th parallel” when conditions allowed.

In other words, the United States would essentially cede North Korean territory to the Chinese military, or let China and South Korea figure out who would control 46,500 square miles of territory and take care of its 25 million occupants, many of whom already do not have enough to eat.

In an interview on other national security issues Friday, a senior administration official who has been deeply involved in the North Korea contingency planning declined to speak about the issue, even to confirm that the conversations the secretary described had taken place.

The White House has been more focused on the beleaguered Tillerson’s public offer to begin talks with North Korea on any issues, even “the weather,” from which he backtracked Friday in a presentation to the United Nations.

But the reference to planning for North Korean collapse, while not drawing wide notice, caught the attention of those who have been drawing up military plans for a number of possible scenarios, including US pre-emptive strikes. Asked whether Tillerson had referred by mistake to entreaties to the Chinese that previous administrations kept secret, Steven Goldstein, the new undersecretary of state for public diplomacy and public affairs, said it was quite deliberate.

“The secretary reiterated the position he has taken in meetings with Chinese counterparts,” he said. “He would like the US and Chinese military leaders to develop a plan for the safe disposition of North Korea’s nuclear weapons were the regime to collapse.” He added: “While the secretary has never advocated for regime change, we all have an obligation to be prepared for any scenario.”

There is no indication that the Chinese have responded, or that military officials have met — though Beijing would almost certainly keep that secret if it occurred.

According to current and former US officials, the contingency plans to seize North Korea’s nuclear arsenal have grown in complexity in recent years, largely because the North Korean arsenal has grown.

David E. Sanger/The New York Times