The production of North Korean weapons-grade nuclear material has more than doubled in the last five years, said leader Kim Jong-un, who promised an "exponential" increase in his country's military nuclear capabilities, local media reported this Thursday.

On a visit to a new nuclear material production plant, Kim said North Korea confirmed “an ambitious future plan designed to strengthen our state's nuclear forces at an exponential rate,” state news agency KCNA reported Thursday.

North Korea sees its nuclear arsenal as a safeguard against an attack by South Korean and US forces.

Kim declared that the increase “signifies an astonishing and successful change that goes beyond any rhetorical description, a historic increase that has marked a momentous milestone in the rapid improvement of our nuclear capabilities.”

North Korea rejects American pressure to give up its nuclear weapons, maintaining that its advance is “irreversible.”

Pyongyang withdrew from the Nuclear Non-Proliferation Treaty in 1993, and since then it has carried out six atomic weapons tests. It is believed to have dozens of nuclear warheads.