South Korea's defence ministry said Tuesday that North Korea appeared to have achieved a "significant" level of technology to miniaturise a nuclear device to be fitted on the tip of a missile.

The ministry made the warning in a white paper due to be released later Tuesday, although defence officials said the nuclear-armed North has yet to demonstrate its miniaturisation capacity.

"North Korea's capabilities of miniaturising nuclear weapons appear to have reached a significant level," the ministry said in a statement.

Pyongyang has conducted three nuclear tests, most recently in February 2013.

The defence ministry said the North had probably secured some 40 kilos (88 pounds) of weapons-grade plutonium by reprocessing spent nuclear fuel rods, and that it is working on a highly enriched uranium programme.