It's the latest indication Pyongyang is getting ready for a third nuclear test, Yonhap reported.
"We've found that work presumed to be part of preparations for a nuclear test has entered its final stage" at the test site in Punggye-ri, said the source, speaking on condition of anonymity.
The site has three known tunnels and the source the activity seen at the tunnel could be meant to draw attention from the more likely test site at another tunnel.
North Korea threatened to detonate a nuclear device after the U.N. Security Council condemned its launch of a long-range rocket Dec. 12.
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