MOSCOW (Reuters). Russian fighter planes, ships and aircraft practice repelling attacks on naval bases, and then respond with air strikes in military drills at the Black Sea. Interfax reported Wednesday that combat exercises were also being held by Ukraine and neighbouring countries.

These drills are occurring at a moment of intense tension in Ukraine. U.S. and Ukrainian officials have expressed concerns over a Russian attack on their southern neighbor, something the Kremlin dismissed as untrue.

Interfax quoted Russia’s Black Sea Fleet as saying that about 10 ships and crews from the Black Sea fleet’s Novorossiysk navy base participated in combat training.

Sukhoi fighter planes were part of the aircrafts that practiced how to deal with enemy air strikes with training flights over Black Sea waters with the Black Sea fleet.

Russia’s Black Sea Fleet has its base in Crimea. This is the Crimea that Moscow annexed to Ukraine in 2014. Kyiv demands Russia return it.

Ukraine accuses Russia of stationing troops close to its border. Belarus has also accused Ukraine of sending migrants through its borders. The operation, consisting military drills, for airborne and anti-tank units, was launched Wednesday by the Ukrainian government.