JOIN USThe work is going on in Punggye district in the northeastern province of North Hamkyong, the region where the North staged tests in 2006 and 2009, Chosun Ilbo quoted intelligence sources as saying.
It said the North is also reportedly speeding up major excavation work and construction of a new building at its main nuclear complex in Yongbyon, north of Pyongyang.
The South's National Intelligence Service could not confirm the report, which comes at a time of increasing international concern over the communist state's nuclear programmes.
The regime revealed an apparently operational uranium enrichment plant at Yongbyon to visiting US experts on November 12. US officials say the new plant could easily be reconfigured to produce weapons-grade uranium, allowing the production of more atomic bombs.
The US State Department said yesterday the North has "at least one other" uranium enrichment site in addition to the one disclosed.
"North Korea is busy digging even in winter when the ground is frozen at two nuclear facilities (in Punggye district and Yongbyon)," Chosun Ilbo quoted an unidentified intelligence official as saying.
Based on an estimate of the amount of soil dug up, the officer speculated that the North has excavated to a depth of more than 500 metres (1,640 feet) in Punggye district.
"If progress goes on at the current pace, the North will have dug a cave one kilometre deep, the depth where it is possible to conduct a nuclear test, between March and May next year," the official was quoted as saying.
Chosun Ilbo also reported major construction work at Yongbyon.