South Korea’s Chosun Ilbo said the information came from a government source.
It relates to preparations for a test in the northeastern town of Punggye-ri, where the North carried out two previous nuclear tests in 2006 and 2009.
A defence ministry spokesman declined to comment on the report.
A South Korean government official told AFP on April 8 on condition of anonymity that satellite images showed a new underground tunnel built at the nuclear test site besides two others where the previous tests were conducted.
“Heaps of earth and sand which had been piled up outside the new tunnel have disappeared,” a government source was quoted as saying by Chosun.
“It is highly likely that the North has installed a nuclear device inside the tunnel and sealed it (with the piles of earth and sand),” the source said.
Since last year, US and South Korean intelligence authorities have been monitoring the excavation by measuring the amount of soil and rocks dumped from the tunnel, it said.
But it has not yet been confirmed whether the North has installed cables for detonation, the source said.
“It is technically feasible for the North to carry out a nuclear test within two weeks,” the source added.
The North, believed to have enough plutonium for six to eight bombs, tested atomic weapons in October 2006 and May 2009. Both were held one to three months after missile tests.
It has vowed to launch satellites “one after another”, vigorously rejecting international condemnation of an April 13 launch that was seen overseas as a disguised ballistic missile test.