Five months before North Korea’s first nuclear test in 2006, U.S. intelligence officials sent a report to Congress warning that secret work also was under way on a biological weapon.

The communist regime, which had long ago acquired the pathogens that cause smallpox and anthrax, had assembled teams of scientists but seemed to be lacking in certain technical skills, the report said.

“Pyongyang’s resources presently include a rudimentary biotechnology infrastructure,” the report by the director of national intelligence explained.

A decade later, the technical hurdles appear to be falling away.

North Korea is moving steadily to acquire the essential machinery that could potentially be used for an advanced bioweapons programme, from factories that can produce microbes by the tonne, to laboratories specialising in genetic modification, according to United States and Asian intelligence officials and weapons experts.

Leader Kim Jong Un’s Government also is dispatching its scientists abroad to seek advanced degrees in microbiology, while offering to sell biotechnology services to the developing world.

U.S. analysts say North Korea could quickly surge into industrial-scale production of biological pathogens if it chooses to do so. Such a move could give the regime yet another fearsome weapon with which to threaten neighbours or U.S. troops in a future conflict, officials and analysts say.

Current and former U.S. officials with access to classified files say they have seen no hard evidence so far that Kim has ordered production of actual weapons, beyond samples and prototypes. And they can only speculate about the reasons.

“That the North Koreans have [biological] agents is known, by various means,” said one knowledgeable U.S. official who, like several others interviewed, spoke on the condition of anonymity. “The lingering question is, why have they acquired the materials and developed the science, but not yet produced weapons?”

But the official, like others interviewed, also acknowledged that spy agencies might not detect a change in North Korea’s programme, since the new capabilities are imbedded within civilian factories ostensibly engaged in making agricultural and pharmaceutical products.

North Korea consistently denies having a biological warfare programme of any kind, and it has worked diligently to keep all evidence of weapons research hidden from sight.

Yet, in 2015, Kim commandeered a crew of North Korean cameramen for a visit to the newly named Pyongyang Biotechnical Institute, a sprawling, two-storey facility on the grounds of what used to a vitamin factory. State-run news media described the institute as a factory for making biological pesticides — mainly, live bacteria that can kill the worms and caterpillars that threaten North Korea’s cabbage crop.

To U.S. analysts studying the video, the images provided an unexpected jolt: On display inside the military-run facility were rooms jammed with expensive equipment, including industrial-scale fermenters used for growing bulk quantities of live microbes, and large dryers designed to turn billions of bacterial spores into a fine powder for easy dispersal.

Many of the machines were banned from sale to North Korea under international sanctions because of their possible use in a bioweapons programme. But Kim, wearing a white lab coat and trailed by a phalanx of scientists and military officers, appeared almost gleeful in showing them off, striking the same rapt pose as when he visits the country’s installations for nuclear weapons and long-range missiles.

“It is hard to avoid the conclusion that the institute is intended to produce military-size batches of anthrax,” Melissa Hanham, a North Korea specialist at the James Martin Centre for Nonproliferation Studies in Monterey, California, wrote in a blog posting after the video was shown.

U.S. analysts now believe the timing of the visit was deliberate: The previous week, on May 28, the Pentagon had publicly acknowledged that live samples of U.S.-made anthrax bacteria had been accidentally shipped to a South Korean military base because of a lab mix-up. North Korea lodged a formal complaint with the United Nations on June 4, calling the incident proof of American “biological warfare schemes” against its citizens.

Kim’s trip to the biotechnology institute came just two days later, and was clearly intended to send a message, Hanham said in an interview.

Some weapons experts were sceptical, noting the absence of biohazard suits and protective gear found in laboratories that work with deadly pathogens. But since the release of the images, subsequent examinations have poked holes in the official story about the factory’s purpose. For one thing, some of the machines shown were not visibly connected to any pipes, vents or ductwork.

Experts also have questioned why North Korea would buy expensive industrial equipment at black-market rates, just to make a pesticide that can be purchased legally, at vastly cheaper rates, from China.

“The real takeaway is that [ North Korea] had the dual-use equipment necessary for bioweapons production,” said Andrew Weber, a former Assistant Secretary of Defence for nuclear, chemical and biological defence programmes. “What the photos show is a modern bio-production capability.”

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That North Korea possesses the basic components for biological weapons is all but settled doctrine within U.S. and Asian military and intelligence establishments, and has been for years.

Although overshadowed by Pyongyang’s nuclear and chemical weapons, the threat of biological attack from the North is regarded as sufficiently serious that the Pentagon routinely vaccinates all Korea-bound troops for exposure to anthrax and smallpox.

But determining North Korea’s precise capabilities — and the regime’s intentions for using such weapons — have been among the toughest intelligence challenges for U.S. analysts.

Questions about North Korea’s capability have taken on a new urgency, as military planners prepare for the possibility that tensions with Pyongyang could lead to war.

While U.S. and South Korean aircraft would seek to knock out suspected chemical and biological facilities from the air, the newest plans include a presumption that infantry divisions would have to face an array of chemical and biological hazards on the battlefield — hazards that may be invisible to fast-moving ground troops, current and former U.S. officials say.

A consensus view among military planners is that Kim is choosing to hold his bioweapons card in reserve for now, while his scientists build up a capacity to manufacture large quantities of pathogens quickly.

Joseph DeTrani, a retired CIA veteran who oversaw intelligence collection for North Korea in the 2000s, noted that ambiguity has been a built-in feature of North Korean weapons programmes for decades.

“They talk openly about their ‘nuclear deterrent,’ but with chemical and biological weapons, it’s different,” DeTrani said. “They’ve always played it close to the vest. For them, it’s a real option. But they want to preserve the possibility of deniability.”