At a think-tank conference on North Korea, Bolton said he could now "speak in unvarnished terms" about the "grave threat" posed by the regime of Kim Jong Un, who has courted Trump.

"It seems to be clear that the DPRK has not made a strategic decision to give up its nuclear weapons," Bolton said, referencing the North's official name, the Democratic People's Republic of Korea.

"In fact, I think the contrary is true. I think the strategic decision that 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," Bolton said at the Center for Strategic and International Studies.

"These are the questions that need to focus our attention -- not can we get another summit with Kim Jong Un."

His comments are at odds with Trump's rosy depictions of Kim after three meetings, with the US leader hailing the young strongman's "beautiful letters" and insisting that Kim would stay true to his word.

Bolton has long been known for his strong opposition to North Korea, which once, before he served with Trump, denounced him as "human scum."

When they parted ways, Trump pointed to a comment by Bolton -- how he favored a "Libyan model" for North Korea -- as an example of his top aide "being not smart."

In 2003 as the United States was invading Iraq, Libyan dictator Moamer Kadhafi negotiated an end to its nuclear program in return for reconciliation with the West.

But Western powers in 2011 backed an uprising against Kadhafi, who was later found in a drainage pipe, tortured and lynched.

Bolton on Monday stood by his remarks, saying the "Libyan model" referred to a strategic decision to give up nuclear weapons.

"We saw Moamer Kadhafi make an unambiguous decision that he and Libya would be better off without developing nuclear weapons," said Bolton, an architect of the Iraq war.

- Seeing threat in launches -

Bolton also took aim at North Korea's repeated firing of short-range projectiles. Trump has played them down, saying that Kim enjoys the launches and that they do not violate agreements.

"The testing of shorter-range ballistic missiles that we've see in recent months doesn't give us any reason to think that those are not threatening," he said.

He said that short-range projectiles would help the North develop the technology behind longer-range missiles.

He faulted the halt to military exercises with South Korea, ordered by Trump as a conciliatory gesture, saying that it weakened military preparedness.

He also said that the United States was paying insufficient attention to tensions between South Korea and Japan, which have soared over issues related to colonial history.

"It's well below the radar screen here in the United States, which is a big mistake for our country not paying more attention to," he said.

Trump announced on September 10 that he had fired Bolton -- who said he quit -- after multiple disagreements.

Besides North Korea, Bolton had pushed for a tough line on Iran and Venezuela, musing in the past about military action.

Bolton at the conference urged a more engaged United States, an implicit criticism of Trump's hopes to scale back US commitments overseas.

"This is not the time for US disengagement or withdrawal. It is a time for more US involvement and leadership on the Korean peninsula, in Asia and worldwide -- more, not less."