Kim recently visited a nuclear-material production facility, Pyongyang's official Korean Central News Agency said, where Kim warned of an "inevitable" confrontation with hostile nations and said 2025 would be a "crucial year" for bolstering North Korea's nuclear forces.

"It is our firm political and military stand and invariable noble task and duty to develop the state's nuclear counteraction posture indefinitely," Kim said, according to KCNA.

The report, and Kim's nuclear factory visit, follow Pyongyang's test-firing on Saturday of sea-to-surface strategic guided cruise missiles, its first weapons test since Trump returned to the White House on January 20.

In response, an official with the US National Security Council said Trump would pursue "the complete denuclearisation of North Korea, just as he did in his first term", according to a report from South Korean Yonhap news agency.

Trump, who had a rare series of meetings with Kim during his first term, said in an interview last week that he would reach out to the North Korean leader again, calling Kim a "smart guy".

Despite enduring crippling economic sanctions, North Korea declared itself an "irreversible" nuclear state in 2022.

Pyongyang says the weapons are necessary for its self-defence and to counter hostilities from Washington.

Yang Moo-jin, president of the University of North Korean Studies in Seoul, said Wednesday that the Trump administation appeared to be "adopting a two-track approach".

"Trump is extending overtures for dialogue with Kim to encourage discussions from a political perspective," Yang told AFP.

"On the other hand, Washington's working-level officials are now making clear that they focused on negotiating with the ultimate goal of achieving complete denuclearisation," he said.

But Yang added that North Korea might still want to speak to Washington "as they do need sanctions relief to keep its regime".

- 'In love' -

During his first term in office, Trump met with Kim three times, beginning with a landmark summit in Singapore in June 2018.

A few months after, Trump famously told a rally of his supporters that the two men had fallen "in love".

But their second summit in Hanoi collapsed in 2019 over sanctions relief and what Pyongyang would be willing to give up in return.

"I think he (Kim) misses me," Trump said in July last year, adding "it is nice to get along with somebody that has a lot of nuclear weapons".

In a commentary released the same month, North Korea said while it was true Trump tried to reflect the two leaders' "special personal relations", he "did not bring about any substantial positive change" during his first term in office.

"Even if any administration takes office in the US, the political climate, which is confused by the infighting of the two parties, does not change and, accordingly, we do not care about this," it added.