WASHINGTON — Secretary of State Rex W. Tillerson let slip last week a few tantalizing 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, American diplomats have been trying to engage their Chinese counterparts in a discussion of this scenario, hoping to avoid a conflict between arriving American 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 American 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 Mr. Kim.

So it was surprising to Mr. 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 American forces landed in North Korea to search for and deactivate nuclear weapons, the troops would do their work and then retreat.