North Korea has repeated its claim to be able to make nuclear warheads small enough to fit on a missile.
A defence official said in a statement on Wednesday that its nuclear programme had "long been in the full-fledged stage of miniaturisation".
However, analysts say while there is evidence the programme is advancing, it is difficult to assess its true extent.
The claim comes hours after North Korea cancelled a planned visit by the UN Secretary General Ban Ki-moon.
Mr Ban was due to visit an industrial complex in the Kaesong economic zone run jointly by the North and South and would have been the first UN chief to visit North Korea in more than 20 years.
Speaking at a forum in Seoul, he said the move was "deeply regrettable" and that no explanation was given.