WASHINGTON — In his first public comments since leaving the White House, John R. Bolton, the former national security adviser, delivered a stark warning on Monday about President Trump’s approach to North Korea, undercutting the president’s insistence that its leader, Kim Jong-un, wanted to make a denuclearization deal with him.

Without mentioning Mr. Trump by name, Mr. Bolton said he wanted to “speak in unvarnished terms about the threat posed by North Korea,” and made it clear that he thought the president’s outreach to Mr. Kim had benefited only one side. And while Mr. Trump has made a deal with Mr. Kim one of his signature foreign policy goals, Mr. Bolton asserted that there had been no gains with his approach.

“The strategic decision Kim Jong-un is operating through is that he will do whatever he can to keep a deliverable nuclear weapons capability and to develop and enhance it further,” Mr. Bolton said during an appearance at the Center for Strategic and International Studies. “Under current circumstances, he will never give up the nuclear weapons voluntarily.”

Mr. Bolton was pushed out from his job as Mr. Trump’s third national security adviser less than a month ago amid disputes over how to handle major foreign policy challenges, including North Korea.