South Korea's defence ministry said Friday it doubted North Korea had the ability to launch a nuclear-armed ballistic missile as claimed in a report by the US military spy agency.

"North Korea has conducted three nuclear tests, but it is still doubtful that North Korea has made a small, light warhead that can be mounted on a missile," ministry spokesman Kim Min-Seok told reporters.

But Kim added that the North was "getting to that stage" where it could miniaturise a warhead for missile deployment.

He was reacting to the findings of the US Defense Intelligence Agency (DIA) that stated Pyongyang could be capable of launching a nuclear warhead, albeit an unpredictable one.

The alarming assessment, revealed by a US lawmaker at a congressional hearing, came as tensions on the Korean peninsula mounted over an expected missile launch by Pyongyang in the coming days.

In his briefing, Kim stressed there had been no discernible build-up or movement of North Korea's armed forces in recent days.

"There is no great change," he said.