KIEV/SEOUL (REUTERS, NYTIMES) - Ukraine denied on Monday (Aug 15) that it had ever supplied defence technology to North Korea, responding to an article in the New York Times that said North Korea may have purchased rocket engines from Ukrainian factory Yuzhmash.

Concern that North Korea is close to achieving its goal of putting the mainland United States within range of a nuclear weapon has raised global tensions in recent weeks. The United States is a key backer of Ukraine.

The secretary of the Ukrainian Security and Defence Council, Oleksandr Turchynov, said Ukraine “has never supplied rocket engines or any kind of missile technology to North Korea.”

State-owned Yuzhmash said it had not produced military-grade ballistic missiles since independence from the Soviet Union in 1991. “In the years of independence, Yuzhmash has not produced, and is not producing, missiles and military missile systems,” it said in a statement published on their company’s website.

The denials came after an expert analysis being published on Monday and classified assessments by US intelligence agencies indicated that North Korea's success in testing a intercontinental missile that appears able to reach the US was made possible by black-market purchases of powerful rocket engines probably from a Ukrainian factory with historical ties to Russia's missile programme.

The studies may solve the mystery of how North Korea began succeeding so suddenly after a string of fiery missile failures, some of which may have been caused by US sabotage of its supply chains and cyber-attacks on its launches.

After those failures, the North changed designs and suppliers in the past two years, according to a new study by Michael Elleman, a missile expert at the International Institute for Strategic Studies (IISS).

Such a degree of aid to North Korea from afar would be notable because President Donald Trump has singled out only China as the North's main source of economic and technological support.

He has never blamed Ukraine or Russia, though his secretary of state, Rex Tillerson, made an oblique reference to both China and Russia as the nation's "principal economic enablers" after the North's most recent ICBM launch last month (July).

Analysts who studied photographs of the North's leader, Kim Jong Un, inspecting the new rocket motors concluded that they derive from designs that once powered the Soviet Union's missile fleet. The engines were so powerful that a single missile could hurl 10 thermonuclear warheads between continents.

Those engines were linked to only a few former Soviet sites. Government investigators and experts have focused their inquiries on a missile factory in Dnipro, Ukraine, on the edge of the territory where Russia is fighting a low-level war to break off part of Ukraine.

During the Cold War, the factory made the deadliest missiles in the Soviet arsenal, including the giant SS-18. It remained one of Russia's primary producers of missiles even after Ukraine gained independence.

But since Ukraine's pro-Russian president Viktor Yanukovych was removed from power in 2014, the state-owned factory, known as Yuzhmash, has fallen on hard times.

The Russians cancelled upgrades of their nuclear fleet. The factory is underused, awash in unpaid bills and low morale.

Experts believe it is the most likely source of the engines that in July powered the two ICBM tests, which were the first to suggest that North Korea has the range, if not necessarily the accuracy or warhead technology, to threaten American cities.

"It's likely that these engines came from Ukraine - probably illicitly," Elleman said in an interview.

"The big question is how many they have and whether the Ukrainians are helping them now. I'm very worried."

Bolstering his conclusion, he added, was a finding by United Nations investigators that North Korea tried six years ago to steal missile secrets from the Ukrainian complex. Two North Koreans were caught, and a UN report said the information they tried to steal was focused on advanced "missile systems, liquid-propellant engines, spacecraft and missile fuel supply systems." Investigators now believe that, amid the chaos of post-revolutionary Ukraine, Pyongyang tried again.

Elleman's detailed analysis is a public confirmation of what intelligence officials have been saying privately for some time: The new missiles are based on a technology so complex that it would have been impossible for the North Koreans to have switched gears so quickly themselves. They apparently fired up the new engine for the first time in September 2016 - meaning it took only 10 months to go from that basic milestone to firing an ICBM, a short time unless they were able to buy designs, hardware and expertise on the black market.

The White House had no comment when asked about the intelligence assessments.

Last month, Yuzhmash denied reports that the factory complex was struggling for survival and selling its technologies abroad, in particular to China. Its website says the company does not, has not and will not participate in "the transfer of potentially dangerous technologies outside Ukraine."

US investigators do not believe that denial, though they say there is no evidence that the government of President Petro Poroshenko, who recently visited the White House, had any knowledge or control over what was happening inside the facility. How the Russian-designed engines, called the RD-250, got to North Korea is still a mystery.

Elleman was unable to rule out the possibility that a large Russian missile enterprise, Energomash, which has strong ties to the Ukrainian complex, had a role in the transfer of the RD-250 engine technology to North Korea.

He said leftover RD-250 engines might also be stored in Russian warehouses.

But the fact that the powerful engines did get to North Korea, despite a raft of U.N. sanctions, suggests a broad intelligence failure involving the many nations that monitor Pyongyang.

Since President Barack Obama ordered an increase in sabotage against the North's missile systems in 2014, US officials have closely monitored their success. They appeared to have won a major victory last fall, when Kim ordered an end to flight tests of the Musudan, an intermediate-range missile that was a focus of the American sabotage effort.

But no sooner had Kim ordered a stand-down of that system than the North rolled out engines of a different design. And those tests were more successful.

US officials will not say when they caught on to the North's change of direction. But there is considerable evidence they came to it late.

Leon Panetta, the former CIA director, said on CBS' "Face the Nation" on Sunday that the North Korean drive to get workable ICBMs that could be integrated with nuclear weapons moved more quickly than the intelligence community had expected.

"The rapid nature of how they've been able to come to that capability is something, frankly, that has surprised both the United States and the world," he said.

It is unclear who is responsible for selling the rockets and the design knowledge, and intelligence officials have differing theories about the details. But Elleman makes a strong circumstantial case that would implicate the deteriorating factory complex and its underemployed engineers.

"I feel for those guys," said Elleman, who visited the factory repeatedly a decade ago while working on federal projects to curb weapon threats. "They don't want to do bad things."

Dnipro has been called the world's fastest-shrinking city. The sprawling factory, southeast of Kiev and once a dynamo of the Cold War, is having a hard time finding customers.

US intelligence officials note that North Korea has exploited the black market in missile technology for decades, and built an infrastructure of universities, design centres and factories of their own.

They have also recruited help: In 1992, officials at a Moscow airport stopped a team of missile experts from travelling to Pyongyang.

That was only a temporary setback for North Korea. It obtained the design for the R-27, a compact missile made for Soviet submarines, created by the Makeyev Design Bureau, an industrial complex in the Ural Mountains that employed the rogue experts apprehended at the Moscow airport.

But the R-27 was complicated, and the design was difficult for the North to copy and fly successfully.

Eventually, the North turned to an alternative font of engine secrets - the Yuzhmash plant in Ukraine, as well as its design bureau, Yuzhnoye. The team's engines were potentially easier to copy because they were designed not for cramped submarines but roomier land-based missiles. That simplified the engineering.

Economically, the plant and design bureau faced new headwinds after Russia in early 2014 invaded and annexed Crimea, a part of Ukraine. Relations between the two nations turned icy, and Moscow withdrew plans to have Yuzhmash make new versions of the SS-18 missile.

In July 2014, a report for the Carnegie Endowment for International Peace warned that such economic upset could put Ukrainian missile and atomic experts "out of work and could expose their crucial know-how to rogue regimes and proliferators." The first clues that a Ukrainian engine had fallen into North Korean hands came in September when Kim supervised a ground test of a new rocket engine that analysts called the biggest and most powerful to date.

Norbert Bruegge, a German analyst, reported that photos of the engine firing revealed strong similarities between it and the RD-250, a Yuzhmash model.

Alarms rang louder after a second ground firing of the North's new engine, in March, and its powering the flight in May of a new intermediate-range missile, the Hwasong-12. It broke the North's record for missile distance. Its high trajectory, if levelled out, translated into about 2,800 miles, or far enough to fly beyond the U.S. military base at Guam.

On June 1, Elleman struck an apprehensive note. He argued that the potent engine clearly hailed from "a different manufacturer than all the other engines that we've seen." Elleman said the North's diversification into a new line of missile engines was important because it undermined the West's assumptions about the nation's missile prowess. "We could be in for surprises."

That is exactly what happened. The first of the North's two tests in July of a new missile, the Hwasong-14, went a distance sufficient to threaten Alaska, surprising the intelligence community. The second went far enough to reach the West Coast, and perhaps Denver or Chicago.

Last week, the Bulletin of the Atomic Scientists featured a detailed analysis of the new engine, also concluding it was derived from the RD-250. The finding, the analysts said, "raises new and potentially ominous questions."

The emerging clues suggest not only new threats from North Korea, analysts say, but new dangers of global missile proliferation because the Ukrainian factory remains financially beleaguered. It now makes trolley buses and tractors, while seeking new rocket contracts to help regain some of its past glory.

President Donald Trump warned at the weekend that the US military was “locked and loaded” if North Korea acted unwisely after threatening last week to land missiles near the US Pacific territory of Guam. North Korean officials in turn have accused the US leader of driving the Korean peninsula to the brink of nuclear war.

On Sunday, the head of the U.S. Central Intelligence Agency said he would not be surprised if North Korea tested another missile, after it conducted two tests in July.