HANOI, Vietnam — In open dispute, the U.S. and North Korea offered contradictory accounts Thursday of why the summit between Donald Trump and Kim Jong Un broke down, though both pointed to punishing American sanctions as a sticking point in the high-stakes nuclear negotiation.

President Trump, on his way back to Washington on Thursday, said before leaving Hanoi that the talks collapsed because North Korea’s leader insisted that all the sanctions the U.S. has imposed on Pyongyang be lifted without the North firmly committing to eliminate its nuclear arsenal.

But North Korea challenged that account, insisting it had asked only partial sanctions relief in exchange for shutting down its main nuclear complex. Foreign Minister Ri Yong Ho commented on the talks during an abruptly scheduled middle-of-the-night news conference after Trump was in the air.

Ri said the North was also ready to offer in writing a permanent halt of the country’s nuclear and intercontinental ballistic missile tests and that Washington had wasted an opportunity that “may not come again.” He said the North’s position won’t change even if the United States offers to resume another round of dialogue.

Trump made no mention of the disagreement as he addressed U.S. troops during a stopover at Joint Base Elmendorf-Richardson in Alaska, though White House spokeswoman Sarah Sanders said he was aware of Ri’s comments.

Instead, Trump focused on U.S. military might and offered a broad warning to U.S. enemies.

“America does not seek conflict, but if we are forced to defend ourselves we will fight and we will win in an overwhelming fashion,” he declared.

Earlier on Thursday in Hanoi, Trump had told reporters the North had demanded a full removal of sanctions in exchange for shutting the Yongbyon nuclear facility. Trump said that there had been a proposed agreement “ready to be signed.” However, he said after the summit was cut short, “Sometimes you have to walk.”

The demise of the talks came after Trump and Kim had appeared ready to inch toward normalizing relations between their still technically warring nations.