Russian military forces are carrying out "logistical exercises" in and around the Crimean Peninsula, the Ukrainian Black Sea region that Russia annexed in 2014.
Russia's state RIA-Novosti agency announced the August 18 drills. The point of the exercise is to train moving troops, armor, and other equipment from Russia to Crimea.
Some 2,500 troops, 350 armored vehicles, a large landing ship, a submarine, and other units are participating.
The current exercises are the forerunner of a much larger war-games exercise planned in the region for September.
The Defense Ministry said Defense Minister Sergei Shoigu observed part of the exercise on August 17 in the Russian port city of Novorossiisk.
Tensions have been high in the region since Moscow earlier this month accused Ukraine of plotting "sabotage" attacks in Crimea. Kyiv denies any such plots.
Recent weeks have also seen a noticeable increase in shelling and other violence in eastern Ukraine, where Kyiv is battling against Russia-back separatists.
Ukrainian President Petro Poroshenko on August 17 warned that he does not rule out "a full-scale Russian invasion" and said that if the situation in the east and around Crimea continues to deteriorate, "we will have to impose martial law and order mobilization."