The declaration comes less than two weeks before the inauguration of Joe Biden as US president, and after a tumultuous relationship between Kim and the outgoing 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 no substantive progress was made, with the process deadlocked after a meeting in Hanoi broke up over sanctions relief and what the North would be willing to give up in return.

Pyongyang "should focus and be developed on subverting the US, the biggest obstacle for our revolution and our biggest enemy", Kim told the five-yearly congress of the ruling Workers' Party of Korea, the official KCNA news agency reported.

"No matter who is in power, the true nature of its policy against North Korea will never change," it quoted him as saying, without mentioning Biden by name.

Pyongyang has poured vast amounts of 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 whole US, at a cost of increasingly stringent international sanctions.

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

"New planning research for a nuclear-powered submarine has been completed and is to enter the final examination process," Kim told the congress.

The country should "further advance nuclear technology" and develop small-sized, lightweight nuclear warheads "to be applied differently depending on target subjects", he added.

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