North Korean leader Kim Jong Un has said unification with the South is no longer possible, and that the constitution should be changed to designate it the "principal enemy".
Mr Kim also said three organisations dealing with reunification would shut down, state media KCNA reported.
South Korea's president said it would respond "multiple times stronger" to any provocation from the North.
The two Koreas have been divided since the Korean War ended in 1953.
They did not sign a peace treaty and therefore have remained technically still at war ever since.
In a speech delivered at the Supreme People's Assembly - North Korea's rubber-stamp parliament - Mr Kim said that the constitution should be amended to educate North Koreans that South Korea is a "primary foe and invariable principal enemy".
He also said that if a war breaks out on the Korean peninsula, the country's constitution should reflect the issue of "occupying", "recapturing" and "incorporating" the South into its territory.
Mr Kim - who replaced his father, Kim Jong-il, as North Korean leader in 2011 - said the North "did not want war, but we also have no intention of avoiding it", according to KCNA.
He said he was taking a "new stand" on north-south relations, which included dismantling all organisations tasked with reunification.
Speaking to his cabinet on Tuesday, South Korean President Yoon Suk Yeol said that if the North carried out a provocation, the South "will retaliate multiple times stronger", pointing to the South Korean military's "overwhelming response capabilities".
Heightened tensions
His comments came as relations significantly weakened on the Korean Peninsula in recent months.
In November, North Korea fully suspended a five-year military deal with the South aimed at lowering military tensions. It promised to withdraw all measures "taken to prevent military conflict in all spheres including ground, sea and air", and said it would deploy more forces to the border region.
The South had partly suspended the agreement days earlier after Mr Kim claimed to have successfully launched a spy satellite into space.
The rhetoric - and provocative actions - from the North have only escalated since then.
At year-end policy meetings, Mr Kim said he needed to "newly formulate" the North's stance towards inter-Korean relations and reunification policy, adding that the stated goal was to "make a decisive policy change" related to "the enemy".
He also threatened a nuclear attack on the South, and called for a build-up of his country's military arsenal.
The North has also launched missiles in recent weeks, as well as live-fire exercises close to South Korean territory.
In a report published last week for 38 North, a US-based organisation with a focus on North Korea, former State Department official Robert Carlin and nuclear scientist Siegfried S Hecker said they saw the situation on the Korean Peninsula as "more dangerous than it has ever been" since the start of the Korean War in 1950.
"That may sound overly dramatic, but we believe that, like his grandfather in 1950, Kim Jong Un has made a strategic decision to go to war," it said.
"We do not know when or how Kim plans to pull the trigger, but the danger is already far beyond the routine warnings in Washington, Seoul and Tokyo about Pyongyang's 'provocations'."
It added that it did not see the "war preparation themes" in North Korean media as "typical bluster".
Meanwhile North Korea's Foreign Minister Choe Son Hui is in Russia where she is expected to meet President Vladimir Putin.
The two countries have boosted ties recently, with both isolated by Western powers, and last September Mr Kim visited Russia where he met Mr Putin.
