Japan's chief cabinet secretary, Katsunobu Kato, said Tuesday at a regular press briefing that Tokyo rejects a nuclear North Korea. At the Party Congress last week, Kim had called for progress in Pyongyang's nuclear weapons buildup.
The North Korean leader had said last week a long-range missile capable of reaching the United States should be improved for accuracy. Kim also claimed a nuclear-powered submarine was under development.
Kato said Tokyo plans to cooperate with stakeholders, according to South Korean news agency Yonhap and YTN.
"In the future, Japan and the United States, Japan, the United States and South Korea, will continue to cooperate closely," the spokesman said.
Kato also said Japan will cooperate with the international community, including China and Russia, to "promote the full implementation of relevant United Nations Security Council resolutions."
Last year in a defense white paper, Japan said North Korea most likely has the capability to launch an attack against Japan with a nuclear warhead.
The government paper also said the Kim regime may have perfected the capability to miniaturize nuclear warheads and mount them on ballistic missiles capable of reaching their targets.
Kato said Japan will not change the way the government addresses the North Korean leader. Kim was conferred the title of general secretary at the Party Congress, but according to Kato, Japan will continue to use the title "chairman of the state affairs commission" to refer to Kim, reports said.
The North Korean leader met with U.S. President Donald Trump three times, but the two sides did not reach an agreement on denuclearization.
Trump has said he maintains a close relationship with Kim, referring to written correspondence with the North Korean head of state. U.S. President-elect Joe Biden, who has pledged to follow through with "principled diplomacy," has slammed Trump for meeting with Kim.
Last week, Kim declared the United States to be North Korea's "biggest enemy," but has stayed away from launching missiles.