The declaration comes less than two weeks ahead of the new US president's inauguration and after a tumultuous relationship between Kim and the outgoing leader Donald Trump.

Kim and Trump first engaged in a war of words and mutual threats, before an extraordinary diplomatic bromance that featured headline-grabbing summits and declarations of love by the US president.

But little substantive progress was made, with the process deadlocked after their February 2019 meeting in Hanoi broke down over sanctions relief and what Pyongyang would be willing to give up in return.

The North should focus on "containing and subduing the US, the fundamental obstacle to the development of our revolution and our foremost principal enemy", Kim told the five-yearly congress of the ruling Workers' Party of Korea, the official KCNA news agency reported.

"The real intention of its policy toward the DPRK will never change, whoever comes into power in the US," it quoted him as saying, using the initials of the North's official name but without specifically mentioning Biden.

"The check has come due on the Singapore and Hanoi Summits," tweeted Ankit Panda of the Carnegie Endowment. "And the Biden administration gets to pick up the tab."

The change of leadership in Washington presents a challenge for Pyongyang, which has previously called Biden a "rabid dog", while he characterised Kim as a "thug" during the presidential debates.

The US is expected to return to more orthodox diplomatic approaches under Biden, such as insisting on extensive progress at working-level talks before any leaders' summit can be considered.

Kim "sees a stalemate that won't change anytime soon", said Harry Kazianis of the Center for the National Interest.

The process with Trump was brokered by South Korean President Moon Jae-in, but Kim accused Seoul of "raising inessential issues (such) as anti-epidemic and humanitarian cooperation" while breaching inter-Korean agreements and ignoring "our repeated warnings" to stop joint military drills with the US.

- Strategic balance -

Pyongyang has poured vast resources into developing its nuclear weapons and ballistic missiles, which it says it needs to defend itself against a possible US invasion.

The programmes have made rapid progress under Kim, including by far its most powerful nuclear blast to date and missiles capable of reaching the entire continental US, at a cost of increasingly stringent international sanctions.

At a military parade in October, it showed off a huge new intercontinental ballistic missile (ICBM) that analysts concurred was the largest road-mobile, liquid-fuelled missile anywhere in the world, and was highly likely to be designed to carry multiple warheads in independent re-entry vehicles.

The North has also completed plans for a nuclear-powered submarine, Kim said -- something that would change the strategic balance.

Such a weapon, if it was built and went into service, could enable Pyongyang to surreptitiously bring its missiles close to the United States, cutting down warning times ahead of any launch.

Designs for the vessel were "in the stage of final examination", Kim said, adding the North was also looking to develop technology including military reconnaissance satellites, hypersonic gliding warheads, and solid-fuel ICBMs.

The Biden administration was unlikely to react strongly to Kim's comments as they were "only words", Cho Seong-ryoul of the Institute for National Security Strategy in Seoul told AFP.

"But if the North carries them into action with provocation or launches, I expect it to respond severely."

- Work report -

Kim's declarations came in his nine-hour work report to the meeting, spread over three days, which KCNA was reporting in detail for the first time.

The congress is the top ruling party gathering, a grand political set-piece that reinforces the regime's authority and can serve as a platform for announcements of policy shifts or elite personnel changes.

For several days, state television has been showing images of the 7,000 delegates and attendees packed into the cavernous April 25 House of Culture venue -- none of them wearing masks -- repeatedly applauding Kim wildly during his speech.

The gathering comes with North Korea more isolated than ever after closing its borders last January to protect itself against the coronavirus that first emerged in neighbour and key ally China.

That has added to the pressures on the North, with Pyongyang blockading itself far more effectively than even the most hawkish advocate of sanctions could ever hope to achieve, and trade with China at a fraction of the usual level.

In his work report, Kim admitted mistakes had been made in the last five years and that "almost all sectors fell a long way short of the set objectives" in the country's economic plan.