But United States intelligence officials have said they suspect the blast 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 partially powered by a small nuclear reactor, eliminating the usual distance limitations of conventionally fueled missiles.

As envisioned by Mr. Putin, who played animated video of the missile at a state-of-the-union speech in 2018, the Skyfall is part of a new class of weapons designed to evade American missile defenses.

In several recent Pentagon and other government reports, the prospect of a Russian nuclear-powered cruise missiles has been frequently cited as a potential new kind of threat. They are launched into the air and able to weave an unpredictable path at relatively low altitudes.