First Lieutenant Anatoly Mozgovoy told Reuters on the phone from inside the base that there were shots fired and that the Ukrainian soldiers were unarmed. Asked if the base had been taken over, he said: "Yes".
Vladislav Seleznyov, a Ukrainian military spokesman in Crimea, earlier said Russian forces had been forcing their way in and were assisted by helicopters.
Russian forces captured part of the base of the 1st Separate Marine Battalion, Ukraine's top military unit, earlier this month, as Russia was seizing all of the Black Sea peninsula.
But Ukrainians had previously appeared to be in control of the armoury, the barracks and other facilities within the compound.