The officials have said nothing publicly about what is 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. And Russia’s story about what happened on 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.

New nuclear missile: U.S. 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 . Some arms experts have doubts about the technology.

Reaction: The Russian government’s slow and secretive response has prompted anxiety in nearby cities and towns — and attracted the attention of analysts who believe the explosion may offer a glimpse of technological weaknesses in Russia’s new arms program.