Russia closes access to three regions of the Black Sea for six months, Russian media outlets say.
The restriction will apply to the coast of Crimea between Sevastopol and Gurzuf and off the coast of the Kerch Peninsula in the area of Cape Opuk and near the western edge of the peninsula.
Exercises of the Russian fleet are planned.
The Russian Federation says these areas are located to the "outer border of the territorial sea of the Russian Federation.''
The restriction will be in force from 21:00 on April 24 to 21:00 on October 31, Russian officials say.
Ukraine has already protested against the decision.