South Korea's Yonhap news agency said North Korea is preparing to defy international sanctions and conduct a fifth nuclear test.
Yonhap said Sunday it has learned from various government sources that activity at the North's Punggye-ri nuclear test site has "increased two- to threefold recently."
The unidentified officials believe trucks seen moving in and out of the site may be carrying nuclear technicians.
Concerns that Pyongyang is about to conduct another nuclear test comes as the regime prepares to hold a rare meeting of its ruling Workers Party in early May.
Leader Kim Jong Un is expected to use the congress to repeat recent claims of advancements in the North's nuclear and ballistic missile programs, including development of a miniaturized nuclear warhead to fit on long-range ballistic missiles.
None of these claims have been verified, and many observers have accused North Korea of exaggerating its advanced military capabilities.
Experts also believe last week's failure of a medium-range ballistic missile to mark the birthday of founding president Kim Il Sung may also prompt the North to carry out another nuclear test.
The United Nations imposed a set of new and stronger sanctions on North Korea in response to Pyongyang's fourth nuclear test in January, and a ballistic missile test the following month.
Some material for this report came from AFP and Reuters.
