President Donald Trump's former National Security Adviser John Bolton never directly mentioned his old boss Monday in his first public remarks since departing the White House, but the fierce Republican firebrand made clear he does not believe in the president's North Korea strategy.

In a detailed repudiation of Trump's North Korea policy, Bolton warned of a "grave and growing threat" from North Korea's nuclear weapons program, condemned the administration for not condemning North Korea's recent spate of ballistic missile tests, and argued that, in his "unvarnished" view, Kim Jong Un and his regime will never give up their nuclear weapons voluntarily.

Instead of Trump's belief that he can personally negotiate with Kim to surrender his nuclear weapons, Bolton said at the Center for Strategic and International Studies in Washington that the U.S. should be considering regime change in Pyongyang, working with China to reunify North and South Korea, or using military force.

"These are questions that need to focus our attention, not, 'Can we get another summit with Kim Jong Un or what the state of staff-level negotiations are to achieve a commitment from North Korea it will never honor," Bolton said.

In fact, he argued, Kim was operating on the strategic decision to "do whatever he can to keep a deliverable nuclear weapons capability and to develop and enhance it further. He may try to get relief from international sanctions, he may make some concessions, but under current circumstances, he will never give up his nuclear weapons voluntarily."

Two summits between Trump and Kim have yielded a vague joint declaration that committed their countries to the "complete denuclearization of the Korean Peninsula" and a change in their relations. But the two sides still don't have a shared definition of that vague term, and the leaders' second meeting in Hanoi, Vietnam, in February ended when North Korea offered only to dismantle its nuclear facilities at Yongbyon, but not its existing nuclear weapons stockpile or its other secret sites, in exchange for economic sanctions being lifted.

Bolton warned there is a "world out there that's ready to fall sucker to that kind of argument" of a partial or interim deal -- something Trump may be now considering to get talks back on track. After he and Kim met at the Demilitarized Zone between North and South Korea in June -- a meeting which Bolton did not attend -- the two sides said they agreed to working-level talks resuming in July. Three months later, those talks still haven't taken place, and there is no meeting scheduled.

When asked by the CSIS Korea Chair Victor Cha whether Trump's "bromance diplomacy" is the best way forward, Bolton offered a no comment. As the audience laughed, Bolton added, "Nice try."

Trump has already hit Bolton, who said he quit while Trump said he was asked to resign, for playing a spoiler role in North Korea talks, in particular after Bolton mentioned the "Libya model" for how to dismantle Kim's nuclear weapons program. Bolton meant that as offering incentives only after a country dismantles its nuclear weapons program, like Libya's Muammar Gaddafi did, but North Korea points to the deposing of Gadhafi and his torture and death years later as a sign giving up nuclear weapons increases the risk to a strongman leader.

"I don't blame Kim Jong Un for what he said after that, and he wanted nothing to do with John Bolton. And that's not a question of being tough. That's a question of being not smart to say something like that," Trump said in the Oval Office the day after Bolton was ousted.

In a shot at Trump, Bolton said Monday the "Libya model" is not "properly understood." But Bolton also relishes his role as North Korea foil: "I am delighted to be here today. I'm also sure the leadership of North Korea is delighted that I'm here today in a private capacity," Bolton said to laughs. "Perhaps they'll be less delighted now that I can speak in unvarnished terms about the grave and growing threat that the North Korean nuclear weapons program poses to international peace and security."

While Trump has downplayed the nearly dozen ballistic missile tests that Kim has conducted since the Hanoi summit, Bolton argued that it undermines U.S. security and sanctions enforcement against Pyongyang: "When you ask for consistent behavior from others, you have to demonstrate it yourself, and when we fail to do that we open ourselves and our policy to fail."

The tests have also allowed North Korea to enhance their missile capability, Bolton said.

And while Trump has halted major U.S. and South Korean joint military exercises and parroted North Korea's complaint that they are "war games," Bolton said the readiness of U.S. forces is now in question and called for a review by the Pentagon or Congress.