A surge in activity at North Korea's atomic test site suggests preparations for a fifth nuclear test are in their final stages, possibly before a key political event in early May, South Korean media reported Sunday.
The frequency of vehicle, workforce and equipment movements at the Punggye-ri site have "increased two to threefold," since last month, Yonhap news agency said, citing multiple government sources.
Officials believe the trucks seen moving in and out of the complex are likely carrying nuclear technicians.
"If they are signs of nuclear test preparations, it seems the preparations are in the final stages," Yonhap quoted one government source as saying.
North Korea is gearing up for a rare and much-hyped ruling party congress early next month, at which leader Kim Jong-Un is expected to take credit for pushing the country's nuclear weapons programme to new heights.
Numerous analysts have suggested the regime might carry out a fifth nuclear test as a display of defiance and strength just before the congress opens.
A successful test might also go some way to erasing the embarrassing failure on Friday of a medium-range ballistic missile test meant to mark the birthday of the nation's founder Kim Il-Sung.
Pyongyang has claimed a series of achievements in recent months, including miniaturising a nuclear warhead to fit on a missile, developing a warhead that can withstand atmospheric re-entry, and building a solid-fuel missile engine.
Tension has been running high on the divided peninsula since the North conducted its fourth nuclear test in January and a rocket launch a month later that was widely seen as a disguised ballistic missile test.
The UN Security Council responded with its toughest sanctions to date, angering the North. It has since made repeated threats of attacks targeting Seoul and Washington.