North Korea's ruler Kim Jong Un wants to increase the production of weapons-grade material for atomic bombs. Only in this way can the ruling Labor Party's plan to "exponentially increase the nuclear arsenal" and continue to manufacture powerful nuclear weapons be implemented, Kim was quoted as saying by the state-controlled media.

Kim visited the National Nuclear Weapons Institute and guided its work in mounting nuclear warheads on ballistic missiles.

During his visit to the institute on Monday, Kim learned about technical details, including the use of new tactical nuclear weapons, it said. The visit came at a time of rising tensions on the Korean Peninsula. After an unprecedented series of missile tests last year, North Korea tested nuclear-capable missiles again this year despite UN bans. The US and South Korea resumed full-scale joint military exercises.

North Korean media published several photos of allegedly small nuclear warheads with the designation "Hwasan-31" during Kim's visit to the institute. Observers in South Korea suspect that North Korea wants to demonstrate that the warheads can be used on different types of missiles. For many years it was uncertain whether the isolated country already had the technology to downsize a warhead to fit ballistic missiles of various ranges. About three years ago, a report by a UN expert panel stated that several countries now assume that North Korea has small nuclear devices that also fit into ballistic missile warheads.