North Korea can mount a nuclear warhead on a medium-range missile, a South Korean official said on Tuesday in a new assessment of the capability of a country that conducted its fourth nuclear test this year.

"We believe they have accomplished miniaturization of a nuclear warhead to mount it on a Rodong missile," said the South Korean official, who has knowledge of South Korea's assessment of the North's nuclear program. The official spoke to a small group of reporters on condition of anonymity.

North Korean leader Kim Jong Un said last month that his country had miniaturized nuclear warheads to mount on ballistic missiles. It was his first direct statement of a claim often made in state media though never independently verified.