ROSTOV-ON-DON, Russia—A Russian military helicopter has crashed into a mountain in the republic of Dagestan, killing all three crewmen aboard.

Interior Ministry spokesman Oleg Zvyagintsev said the crash occurred Saturday near the village of Botlikh and that witnesses said the Mi-35 helicopter appeared to have become disoriented in foggy conditions.

Dagestan has a heavy military and police presence because of a persistent Islamic insurgency. There were no immediate indications that the helicopter had been attacked.