The declaration -- delivered on Monday -- follows Kim's reappointment a day earlier as head of the authoritarian nation's highest policymaking body, the State Affairs Commission.

"We will continue to firmly consolidate our status as a nuclear-armed state as an irreversible course, while aggressively stepping up our struggle against hostile forces," he said in a policy speech at the rubber-stamp legislature in Pyongyang.

In a lengthy policy address reported by the official Korean Central News Agency, the third-generation leader addressed a wide range of issues, from nuclear weapons and defence policy to economic goals and relations with South Korea and the United States.

"We will, in line with the mission entrusted by the Constitution of the Republic ...., further expand and advance our self-defensive nuclear deterrent," he said, referring to nuclear weapons.

Pursuing an expansion of nuclear weapons to consolidate its status as a nuclear-armed state has been "entirely justified", he added.

The isolated country will ensure "precise readiness" of its nuclear forces, he said, to fend off "strategic threats".

Kim did not mince words about his southern neighbour, which he called "the most hostile state".

"We will designate South Korea as the most hostile state and deal with it by thoroughly," he said.

Pyongyang will "make it pay mercilessly -- without the slightest consideration or hesitation -- for any act that infringes upon our Republic," he added.

Kim is the third-generation ruler of the state founded by his grandfather Kim Il Sung in 1948, and has ruled the country since his father's death in 2011.

The North's legislature re-elected Kim as president of state affairs, KCNA reported earlier, without saying whether the decision was unanimous or with dissent.

Critics argue that elections in North Korea are pre-determined and designed to give the leadership a veneer of democratic legitimacy.