North Korea has adopted a constitutional amendment that says the country will develop nuclear weapons to a higher level as a responsible nuclear power.

North Korea's ruling Workers' Party newspaper Rodong Sinmun reported that the country's Supreme People's Assembly wrapped up its two-day session on Wednesday, with leader Kim Jong Un in attendance.

The newspaper said on Thursday that the assembly had adopted an amendment to clarify North Korea's policy on nuclear force in its constitution.

The amendment says the country will bolster its nuclear arsenal to "deter war and protect regional and global peace."

Kim stressed in his speech that his country's nuclear force will be preserved and strengthened as its permanent strategic asset that no one should be allowed to flout at any time.

Referring to the three-way security cooperation of Japan, the United States and South Korea, Kim said "the emergence of the Asian version" of NATO is the "worst actual threat," as it is "the root cause of war and aggression."

Kim vowed to diversify the North's nuclear strike capability and deploy its nuclear arsenal with different branches and units of the armed forces.

North Korea declared itself a nuclear power in its constitution in 2012.

In September last year, the Supreme People's Assembly adopted a law stipulating conditions for the use of nuclear weapons.

The country made clear it does not rule out the preemptive use of nuclear arms.