John Bolton has comprehensively repudiated the Trump administration’s North Korea policy, warning that the Pyongyang regime is becoming more dangerous with every passing day.

Bolton’s comments on Monday were his first public remarks since leaving his post as national security adviser three weeks ago – but he did not address questions about why he spent 17 months in an administration whose policy he vehemently disagreed with.

At a discussion on North Korea hosted by the Centre for Strategic and International Studies in Washington, Bolton said he was observing a “self-imposed restriction” on talking about his time in office. He was not asked about any of the Ukraine-related issues now at the centre of impeachment proceedings against the president.

Without mentioning Trump by name, Bolton contradicted all the main elements of his policy on North Korea. That policy has been one of personal diplomacy and the claim that Trump’s three summit meetings with Kim Jong-un had led to substantial national security benefits – through the suspension of nuclear and long-range missile testing, and Kim’s undertaking to “denuclearise”.

Saying he could now talk in “unvarnished terms” Bolton dismissed each pillar of the policy in turn. The US was now more at risk rather than becoming safer, he said.

“Every day that goes by makes North Korea a more dangerous country,” he said. “You don’t like their behavior today? What do you think it’ll be like when they have nuclear weapons that can be delivered to American cities? You want to wait till then to act?”

The absence of nuclear and long-range missile testing was not a good thing, Bolton said. It was instead a red flag, showing that North Korea was satisfied it had produced a deliverable nuclear weapon.

He argued: “North Korea has, in its judgment, finished testing, and can produce nuclear warheads and long-range ballistic missiles.

“That’s not an encouraging sign. That’s a sign to be worried about.”

He added: “Moreover, the testing of shorter-range ballistic missiles, that we’ve seen in recent months, doesn’t give us any reason to think that those are not threatening, because the capabilities the technology of short-range ballistic missiles, by definition, can be adapted to the longer-range ballistic missiles.”

Trump and his secretary of state, Mike Pompeo, have repeatedly pointed to Kim’s signature on a joint statement at the first summit in Singapore in June 2018, committing to “work toward complete denuclearisation of the Korean peninsula”, as evidence of his willingness to disarm.

North Korea experts pointed out at the time that Kim was employing a form of words that did not mean unilateral disarmament in the eyes of the regime.

Bolton, who had been the national security adviser for two months at the time of the Singapore summit, said on Monday: “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.”

Furthermore, Bolton said international sanctions would not push Kim to change his mind because “they are not being enforced effectively”. He pointed to evidence of ship-to-ship transfers of coal, oil and other commodities, as ways North Korea and its suppliers were able to cheat.

Bolton said: “If you’re going to have sanctions, make them effective. And if you’re not, then get a different policy.”

He added: “These are questions that need to focus our attention, not can we get another summit with Kim Jong-un.”

Bolton resumed his call for the US to consider military action against North Korea. He rejected the argument that it is not a realistic option as it would trigger a devastating, and quite possibly, nuclear conflict.

“At some point, military force has to be an option,” he said.

Bolton declared that allowing North Korea to develop its capabilities would lead it to sell bombs to others, becoming “the Walmart or the Amazon of deliverable nuclear weapons”.

He has insisted in texts to journalists he resigned from the Trump administration, because of multiple disagreements about policy. Trump has claimed he fired Bolton, and has sought to portray him as a warmonger who helped take the US into the Iraq conflict.

Bolton’s position outside the administration will be a worry for those left inside, and not just because of testimony he might provide in impeachment proceedings. He is assertively undermining Trump’s proudest foreign policy achievement, that the president had hoped to reinforce with a fourth summit, during the 2020 election campaign.