Five months before North Korea's first nuclear test in 2006, US intelligence officials sent a report to Congress warning that secret work also was underway 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 US and Asian intelligence officials and weapons experts. Meanwhile, 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.

The gains have alarmed US analysts, who say North Korea - which has doggedly pursued weapons of mass destruction of every other variety - 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 US troops in a future conflict, officials and analysts say.

Current and former US 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 US official who, like several others interviewed, spoke on the condition of anonymity regarding sensitive military assessments. “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.

“If it started tomorrow we might not know it,” the official said, “unless we're lucky enough to have an informant who happens to be in just the right place.”

In a country that is famously secretive, it is perhaps the most carefully guarded secret of all. 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, the country's leader took it upon himself to partially roll back the curtain. On 6 June of that year, Kim commandeered a crew of North Korean cameramen for a visit to the newly named Pyongyang Biotechnical Institute, a sprawling, two-story 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. But to US 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 was the first public confirmation of the existence of such machines in North Korea, and some US and Asian experts saw their presence as deeply ominous.

“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 Center for Nonproliferation Studies in Monterey, California, wrote in a blog posting after the video was shown. “Regardless of whether the equipment is being used to produce anthrax today, it could be in the near future.”

US analysts now believe the timing of the visit was deliberate: The previous week, on 28 May, the Pentagon had publicly acknowledged that live samples of US-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 4 June, 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.

“Responding by showing their own capability could be taken as a threat,” she said.

Some weapons experts were sceptical, noting the absence of biohazard suits and protective gear typically 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 in the video 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.”

That North Korea possesses the basic components for biological weapons is all but settled doctrine within US 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.

“It's a presumption that they have it and will use it,” said a retired military officer who oversaw troops on the peninsula. “We've had to spend a lot of time figuring out how to deal with some of the WMD.”

But determining North Korea's precise capabilities - and the regime's intentions for using such weapons - have been among the toughest intelligence challenges for US analysts. Official assessments by US and South Korean intelligence agencies have generally concluded that Pyongyang has experimented with a handful of bacterial strains, including the microbes that cause anthrax, cholera and plague. US analysts also have believed since at least the mid-1990s that North Korea possesses the smallpox virus, a conclusion based in part on the discovery of antibodies in the bloodstreams of North Korean soldiers who escaped to the South in the 1980s and 1990s.

That assessment, while controversial, is buttressed by senior North Korean government and military defectors as well as foreign governments with special insight into the regime's military secrets. In 1993, the head of the Russian intelligence agency's foreign branch revealed in a report that North Korea was performing “applied military-biological research” on four pathogens, including microbes that cause anthrax and smallpox.

But more recently, 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 US officials say.

But germs as military weapons also have distinct disadvantages, as they are difficult to control and can take hours or days to kill or disable. 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. Now that the North is equipped with state-of-the-art factories and teams of trained specialists, that shift could conceivably happen in weeks or even days, said the senior official familiar with military preparations for a biological attack.

“The capabilities - the science and technology - all of that now exists,” the official said. “Kim has chosen not to deploy at this time. But ultimately it comes down to a political decision.”

In the waning years of the Cold War, Soviet weapons scientists laboured in secret to build new super-germs more dangerous than those found in nature. With mixed success, using techniques still novel in the 1980s, they spliced together bits of DNA to increase virulence - so that microbes would kill more quickly - or to introduce stealthy features that would make them harder to detect.

There is no known evidence that Pyongyang is working to engineer designer bugs, US analysts say. But there are signs that North Korea is attempting to catapult itself into the 21st century worlds of genetic research and biomedical science.

In 2015, as North Korea's new microbe-producing factory was coming online, North Korean scientists were teaming up with Chinese counterparts on a research project to identify previously unknown bacterial species discovered in the glacial ice in Svalbard, the Norway-owned island chain far north of the Arctic Circle. In a rare instance in which North Koreans took the lead on a peer-reviewed scientific paper, the scientists described using DNA sequencing techniques to isolate the novel strains.

The project was the most dramatic example of what private researchers describe as a surge of interest by North Koreans in genetic engineering and other biotech disciplines. Earlier this year, the Welsh artificial intelligence firm Amplyfi conducted a search of the “deep Web” - the parts of the Internet invisible to the public - for evidence of North Korean interest in bio-defence topics. The company's DataVoyant search tool produced hundreds of thousands of hits and showed a spike in interest in such terms as “gene expression” and “nucleic acid sequence,” beginning two years ago.

A preliminary analysis suggested a pattern of behaviour that US and Asian officials have independently confirmed: a broad North Korean effort to obtain outside expertise from private companies, academic institutions and even non-profits, company officials said. North Korea is believed to have used technical designs from a British agricultural nonprofit in building its microbe-producing Pyongyang Biotechnology Center, and it has sought to enrol promising microbiology students in top research universities across Europe and Asia. In recent years, the North Koreans also have sought to sell medical services to the developing world, in one instance building and staffing an entire hospital in Zambia.

“Every continent is represented,” Amplyfi co-founder Chris Ganje said in a phone interview from the company's headquarters in Cardiff, Wales. He said the search turned up “worrying indicators of unintended support,” adding: “It is obvious that the international community and larger institutions need to be cautious in providing seemingly benign academic scientific education and training to North Korea.”

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A harder challenge is separating legitimate efforts to improve North Korea's medical infrastructure with more sinister attempts to a create new varieties of killing machines, officials and experts acknowledge. 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.”