Russia temporarily restricted flights of aviation over parts of Crimea and the Black Sea, according to a notice to airmen (NOTAM).
"The area has been declared temporarily dangerous for aircraft flights," the NOTAM document said.
The zone of restriction of flights includes some areas over the southern part of Crimea, from Sevastopol to Feodosia, the territorial waters adjacent to the Southern coast of Crimea, and part of the international waters of the Black Sea.
On April 14, Russia announced that it suspended the possibility of foreign warships passage through its territorial waters in the Black Sea from April 24 to Oct. 31.
The move came a day after Russian Defense Minister Sergey Shoygu said Russia "is taking measures in response to threats from NATO."
Responding to the protest from Kyiv, which demanded to cancel the limitations, Deputy Foreign Minister Sergey Ryabkov said no restrictions were introduced for the use of Ukrainian ports on the Sea of Azov and for its trade and civil shipping in the region.
But the US State Department qualified Russia's move as "an escalation."
In recent weeks, Russia has gathered combat-ready forces close to the Ukrainian border which is considered as the largest massing of Russian troops since the illegal annexation of Crimea in 2014.