North Korea's apparent nuclear test on Tuesday had an explosive yield of between six and seven kilotons, South Korea's defence ministry said, revising its earlier estimate of 10 kilotons or more.

Ministry spokesman Kim Min-Seok said seismic monitors had detected a tremor with a 4.9 magnitude emanating from the North's nuclear test site.

"Converting that, we calculate a yield of 6.0-7.0 kilotons," he said.

Kim had earlier put the yield at 10 kilotons or more, signalling "enormous destructive power." (AFP)