Nuclear experts believe North Korea might have secretly obtained components and material from several sources to help it set up a centrifuge facility at its main nuclear complex.
North Korea probably needed external assistance to build a uranium enrichment site, which could offer it a second source of weapons-grade nuclear material, but from whom and where remains shrouded in mystery.
Nuclear experts believe North Korea might have secretly obtained components and material from several sources to help it set up a centrifuge facility at its main nuclear complex, which Washington has condemned as a provocative move by Pyongyang.
A US-based think-tank, the Institute for Science and International Security (ISIS), had said in a report last month that North Korea had used China either directly or indirectly, as a transshipment point, to procure items for enrichment.
"Most believe that China views North Korea's nuclear weapons programme as destabilising to the region," the report said.
"Nonetheless, China is not applying enough resources to detect and stop North Korea's illicit nuclear trade."
ISIS stressed there was no evidence that Beijing was "secretly approving or willfully ignoring exports" to its neighbour to strengthen the North's nuclear weapons programme.
Mark Fitzpatrick, proliferation expert at the International Institute for Strategic Studies in London, said he believed Chinese private firms and individuals, rather than state authorities, may have assisted Pyongyang.
"Chinese middlemen, undoubtedly, are a major part of North Korea's procurement network," he said.
Beijing, the closest thing to an ally for Pyongyang, has not commented on the latest reports on the North's nuclear advances.
For Shannon Kile of the Stockholm International Peace Research Institute (SIPRI), it is unlikely that North Korea had the indigenous expertise needed to assemble a centrifuge plant.
"I just can't imagine they would have been able to do this on their own. This is pretty esoteric technology," he said.
Centrifuges are finely calibrated cylindrical devices that spin at supersonic speed to increase the fissile element in uranium so that it can serve as fuel for nuclear power plants or, if refined to a much higher degree, for atomic bombs.
US nuclear scientist Siegfried Hecker of Stanford University revealed at the weekend he had been shown more than a thousand centrifuges during a tour of the Yongbyon nuclear complex this month.
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