North Korea claims it made progress on nuclear miniaturization by creating warheads small enough to fit on a missile.

A North Korean defense official said in a statement on May 20 that its nuclear program had “long been in the full-fledged stage of miniaturization”.

However, analysts say while there is evidence the program 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.

Ban Ki-moon 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, Ban Ki-moon said the move was “deeply regrettable” and that no explanation was given.

North Korea previously claimed it had miniaturized a device for the nuclear test it conducted in 2013 but experts have continued to debate how far along that process it is.

The latest announcement on nuclear advances follows the publication earlier this month of pictures apparently showing a missile being launched from a submarine. Some experts have said the images may have been doctored.