WASHINGTON • US 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 US would try to race inside North Korea to seize its nuclear weapons if it ever saw evidence that Mr 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 US Special Forces - who have been practising 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 had tried to engage them in joint planning.

The Chinese feared that if news of such a conversation leaked, Beijing would be seen as conspiring with the US 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 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.

American officials have envisioned that North Korean officers, fearing the end of Mr 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 last week, speaking from note cards, Mr Tillerson said at a conference on the Korea crisis that the US and China "have had conversations... 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 Demilitarised Zone," the dividing line between North and South, he said. But if the US' 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 US would essentially cede North Korean territory to the Chinese military, or let China and South Korea figure out who would control 120,434 sq km of territory and take care of its 25 million occupants, many of whom already do not have enough to eat.

The disclosure had been overshadowed by Mr Tillerson's remarks that Washington was willing to talk with Pyongyang without preconditions - a statement that he backed away from on Friday.

NYTIMES, AGENCE FRANCE-PRESSE