NEW YORK • US intelligence officials are racing to understand a mysterious explosion that released radiation off the coast of northern Russia last week, apparently during the test of a new type of nuclear-propelled cruise missile hailed by President Vladimir Putin as the centrepiece of Moscow's arms race with the United States.
US officials have said nothing publicly about the blast last Thursday, possibly one of the worst nuclear accidents in Russia since Chernobyl, although apparently on a far smaller scale, with at least seven people, including scientists, confirmed dead. But the Russian government's slow, secretive response has set off anxiety in nearby cities and towns - and attracted the attention of analysts in the US and Europe who believe the explosion may offer a glimpse of technological weaknesses in Russia's new arms programme.
The accident happened offshore from the Nenoksa Missile Test Site. It was followed by what local officials initially reported was a spike in radiation in the atmosphere.
Late Sunday night, officials at a research institute that had employed five of the scientists who died confirmed for the first time that a small nuclear reactor had exploded during an experiment in the White Sea.
Dr Vyacheslav Solovyov, the scientific director of the Russian Federal Nuclear Centre, said in a video interview with a local newspaper that the institute had been studying "small-scale sources of energy with the use of fissile materials".
But US intelligence officials suspect the explosion involved a prototype of what Nato calls the SSC-X-9 Skyfall. That is a cruise missile that Mr Putin has boasted can reach any corner of the earth because it is partly powered by a small nuclear reactor, eliminating the usual distance limitations of conventionally fuelled missiles.
As envisioned by Mr Putin, the Skyfall is part of a new class of weapons designed to evade US missile defences. Yet for all the hype, Russia's early tests of the cruise missile have all appeared to fail, even before last week's disaster. And Russia's story about what happened last Thursday in the sea off one of its major missile test sites has changed over the past four days as the body count has risen.
Many outside arms experts have long regarded Mr Putin's effort as part fantasy, using a technology the US tried and failed to make work in the 1950s and 1960s. If so, it may call into question one of the Trump administration's justifications for major new spending on US nuclear weapons to counter the Russian build-up - though Washington also cites a parallel programme underway in China.
As envisioned by Mr Putin, the Skyfall is part of a new class of weapons designed to evade US missile defences. Yet for all the hype, Russia's early tests of the cruise missile have all appeared to fail, even before last week's disaster. And Russia's story about what happened last Thursday in the sea off one of its major missile test sites has changed over the past four days as the body count has risen.... US intelligence officials ... are now exploring whether the small nuclear reactor... failed, or exploded.
The accident came at a critical moment in the revived US-Russia nuclear competition. This month, the US withdrew from the Intermediate Nuclear Forces agreement, citing long-running Russian violations, and there are also doubts that the New Strategic Arms Reduction Treaty, the one remaining major treaty limiting nuclear forces, will be renewed before it runs out in less than two years.
To Russian military officials, one of the appeals of the new class of hypersonic and undersea nuclear weapons is that they are not prohibited by any existing treaties - giving them free run to test and deploy them.
The Russian nuclear energy company Rosatom last Saturday said the failure occurred in an "isotope power source for a liquid-fuelled rocket engine". Although the wording was confusing, it was the first official acknowledgement that the accident was nuclear in nature.
The change in Russia's account, along with separate US intelligence reporting and satellite imagery, got the attention of US intelligence officials. They are now exploring whether the small nuclear reactor that Mr Putin talked about when promoting the weapon failed, or exploded. It has never been clear just how far along Mr Putin's grand plans for the cruise missile - called the 9M730 Burevestnick by the Russians - had gotten.
A missile defence review published by the Pentagon notes that "Russian leaders also claim that Russia possesses a new class of missile" that travels five times faster than the speed of sound and moves "just above the atmosphere" in an evasive pattern that would defeat US anti-missile technology. But the report made no assessment of whether they would work.
When Mr Putin first spoke about the new weapons last year, most of the attention fell on his description of an undersea drone, called the Poseidon, that could operate autonomously and, American officials feared, hit the country's west coast in a nuclear "second strike" after an initial exchange.
The Poseidon undersea drone still appears to be years away. But for Mr Putin, the most promising weapon has been the nuclear-propelled cruise missile, which he advertised as being able to fly an unlimited range - an answer to US "global strike" weapons that are designed to reach any corner of the earth, with a non-nuclear warhead.