MOSCOW — A mystery explosion at a Russian weapons testing range involved radioactive materials, the authorities admitted on Saturday, as the blast’s admitted death toll rose and signs of a creeping radiation emergency, or at the least fear of one, grew harder to mask.

In a statement released at 1 a.m. Saturday, Russia’s nuclear energy company, Rosatom, said five employees had died, in addition to the two military personnel previously confirmed dead, as a result of a test on Thursday morning involving “isotopic sources of fuel on a liquid propulsion unit.”

“A bright memory of our comrades will forever live in our hearts,” the statement said.

The statement, though, shed little light on exactly what detonated on Thursday at the White Sea testing range. No use for the propulsion unit was mentioned, although President Vladimir V. Putin previously boasted that Russia has developed a nuclear engine for long-range missiles. And there was no explanation why the authorities in a nearby city had reported rising radiation levels for a brief period several hours later.

While the government has provided no full explanation of what happened, Rosatom’s statement suggested a mishap during a test of a new class of nuclear-engined weapons that Mr. Putin first spoke publicly about last year.