• Last month Pyongyang put a spy satellite into space - and claims it has since photographed major US and South Korean military sites.
• Speaking at an end-of-year meeting of the ruling Workers' Party of Korea, Mr Kim said unification with South Korea was now no longer possible.
North Korea plans to launch three more spy satellites next year as part of efforts to ramp up its military, the country's state media has said.
Last month Pyongyang put a spy satellite into space - and claims it has since photographed major US and South Korean military sites.
Setting out his aims for 2024, Kim Jong Un also said his dealings with South Korea would see "fundamental change".
And he said he had no option but to press ahead with his nuclear ambitions.
Speaking at an end-of-year meeting of the ruling Workers' Party of Korea, Mr Kim said unification with South Korea was now no longer possible. He said Seoul treats his country as an enemy.
It appears to be the first time Mr Kim has said such a thing and marks an official change in policy - although in practice there has been little prospect of unification for some years, with no progress and little effort being made.
Relations between the two countries are in a poor state. Last month, following the spy satellite launch, Pyongyang ripped up a deal with Seoul that was aimed at lowering military tensions.
North Korea also continued regular tests of its missiles throughout 2023 - and earlier this month fired its most advanced long-range missile, defying UN curbs.
The launch of the intercontinental ballistic missile - which has the range to reach the North American continent - drew immediate condemnation from the West.
Meanwhile North Korea is unhappy over South Korea ramping up defence cooperation with the US, after a US submarine armed with nuclear weapons arrived in its waters.
Speaking on Sunday, Mr Kim hit out at the US, saying, according to state media: "Because of reckless moves by the enemies to invade us, it is a fait accompli that a war can break out at any time on the Korean peninsula."
He said South Korea had been transformed into a "forward military base and nuclear arsenal" of the US, and added: "If we look closely at the confrontational military actions by the enemy forces... the word 'war' has become a realistic reality and not an abstract concept."
Mr Kim said 2024 would see more military development, including strengthening nuclear and missile forces and building drones.
"We must respond quickly to a possible nuclear crisis and continue to accelerate preparations to pacify the entire territory of South Korea by mobilising all physical means and forces, including nuclear force, in case of emergency," he said.