The incidents come a day after dozens of pro-Moscow gunmen seized government buildings in the Crimean capital of Simferopol including the regional parliament, which subsequently voted to hold a referendum on May 25 to expand the region's autonomy from Kiev.
"I consider what is happening to be an armed invasion and an occupation," Interior Minister Arsen Avakov wrote in a statement on his Facebook page.
Avakov said that troops from Russia's Black Sea fleet -- stationed in Russian-speaking Crimea -- had blockaded the Belbek airfield near Sevastopol, where Ukrainian soldiers and border guards were stationed.
No armed clashes had occurred but the base was not operating as normal.
Despite not displaying any identifying insignia, the alleged Russian troops "did not hide who they belong to," Avakov wrote.
Meanwhile Avakov said some 120 armed men in camouflage fatigues but without insignia, whom he also said belonged to the Russian armed forces, had entered a second airport in Simferopol early Friday.
The incidents followed the ouster of Kremlin-allied Ukraine president Viktor Yanukovych last weekend in an avalanche of change following the deaths of scores of people in clashes in Kiev.