North Korea has enshrined its status as a nuclear power in its constitution, with leader Kim Jong Un calling for more modern atomic weapons to counter the threat from the United States, state media reported on Thursday.

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 Korean Central News Agency.

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

The United States has "maximised 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 modernisation 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, said.

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