Despite international sanctions over its nuclear weapons programme, North Korea has conducted a record number of missile tests this year, ignoring warnings from the United States, South Korea and their allies.

Diplomatic efforts to convince Pyongyang to give up its atomic arsenal failed, and after Kim's declaration last year that North Korea was an "irreversible" nuclear weapons state, the status was included in the constitution this week.

North Korea's "nuclear force-building policy has been made permanent as the basic law of the state, which no one is allowed to flout", Kim said at a meeting of the State People's Assembly, according to the official Korean Central News Agency.

The rubber-stamp parliament met on Tuesday and Wednesday.

Kim said North Korea needed nuclear weapons to counter an existential threat from the United States and its allies.

The United States has "maximized its nuclear war threats to our Republic by resuming the large-scale nuclear war joint drills with clear aggressive nature and putting the deployment of its strategic nuclear assets near the Korean peninsula on a permanent basis", he said.

Kim described the recently enhanced security cooperation between Washington, Seoul and Tokyo as the "worst actual threat".

As a result, he added, "it is very important for the DPRK to accelerate the modernization of nuclear weapons in order to hold the definite edge of strategic deterrence."

Kim also "stressed the need to push ahead with the work for exponentially boosting the production of nuclear weapons and diversifying the nuclear strike means", according to KCNA.

Enshrining North Korea's nuclear status in the constitution has further dimmed hopes of convincing its leadership to give up atomic weapons, experts said.

"Kim's speech... signifies the permanence of his nuclear force," Yang Moon-jin, president of the University of North Korean Studies in Seoul, told AFP.

"This pushes the prospect of North Korea's denuclearisation even further away."

- Space, missile programmes -

North Korea's weapons tests this year included intercontinental ballistic missiles, and its military this month conducted what it described as simulations of a "tactical nuclear attack".

Pyongyang has also tried and failed twice this year to put a military spy satellite in orbit.

South Korea and the United States have ramped up their security cooperation in response, with large-scale joint drills and trilateral naval exercises with Japan.

The last known North Korean weapons test, involving two short-range ballistic missiles, took place while Kim was on his way to meet President Vladimir Putin in Russia.

Kim's visit to Russia -- his first abroad since the coronavirus pandemic -- fanned Western fears that Moscow and Pyongyang will defy sanctions and strike an arms deal.

Russia is believed to be interested in buying North Korean ammunition for its war in Ukraine, while Pyongyang wants Russian help with its missile and space programmes.

Kim's Russia visit "and the potential strengthening of military cooperation (with Moscow) indicate an increased dedication towards branding itself into a formidable nuclear power", said Yang at the University of North Korean Studies.