Kim said Pyongyang's status as a nuclear-armed state was irreversible and expanding a "self-defensive nuclear deterrent" was essential to national security, regional stability and economic development, state media KCNA reported.
He rejected the idea that nuclear disarmament could be exchanged for economic benefits or security guarantees, saying North Korea had already proven that maintaining nuclear forces while pursuing development was the correct strategic choice.
Nuclear weapons had deterred war and allowed the state to focus resources on economic growth, construction and living standards, he said in the address on Monday to the Supreme People's Assembly, the communist-run country's rubber-stamp legislature.
Kim accused the United States and its allies of destabilising the region by deploying strategic nuclear assets near the Korean peninsula, but said North Korea no longer viewed itself as a country under threat and possessed the power to threaten others if necessary.
Kim said South Korea had been "recognised as the most hostile state" and warned Seoul that any attempt to infringe on North Korea's sovereignty would be met "mercilessly without hesitation or restraint".
The comments are the latest sign of Pyongyang's hardening stance toward Seoul since Kim dropped decades of policy seeking peaceful reunification and moved to redefine relations with the South as those between two hostile states.
Analysts have been watching for any sign that this shift had been codified in law. The state media report did not elaborate.
Alongside security policy, Kim outlined economic priorities, calling on officials to fully implement a new five-year development plan focused on modernising industry, boosting electricity and coal production, increasing food output and expanding housing construction nationwide.
North Korea is one of the world's poorest countries, with a heavily sanctioned economy and chronic shortages that have left much of its population dependent on state rations and informal markets, according to international assessments.
The parliamentary session adopted amendments to the constitution, and passed legislation endorsing the new five-year economic plan, KCNA said.
Politicians also approved a 2026 state budget that raises defence spending to 15.8 per cent of total expenditure, with funding explicitly allocated to expanding nuclear deterrence and war-fighting capabilities, according to a separate budget report released at the session.
