Seoul, Sep 28 : North Korea has stipulated the policy of strengthening its nuclear force in the constitution at a key parliamentary meeting, as leader Kim Jong-un said trilateral security cooperation between South Korea, the US, and Japan poses "the worst actual threat", state media reported on Thursday. The decision was made at the ninth session of the 14th Supreme People's Assembly (SPA) held on Tuesday and Wednesday, with Kim's attendance, according to the North's official Korean Central News Agency (KCNA). At the session, the North unanimously decided to "supplement Article 58 of Chapter 4 of the Socialist Constitution" to ensure the country's right to existence and development, deter war and protect regional and global peace by rapidly developing nuclear weapons to a higher level, Kim said. At an SPA meeting in September last year, North Korea enacted a new nuclear law authorizing the preemptive use of nuclear arms, calling its status of a nuclear state "irreversible", reports Yonhap News Agency. Kim also stressed the need to "exponentially" bolster the country's nuclear arsenal and diversify its nuclear strike capability as well as deployment with different branches and units of the armed forces. He also vowed to strengthen solidarity with countries standing against the US. His message came as North Korea has been seeking to bolster its ties with China and Russia vis-a-vis the strengthening of security cooperation among Seoul, Washington and Tokyo. Earlier this month, Kim also traveled to Russia on his armored train for talks with Vladimir Putin amid concerns about a possible arms deal between the two nations. /IANS