After a series of close encounters between Russian and NATO military aircraft over the Black Sea in recent months, the waterway is now seeing a ramping up of naval activity as well. According to a statement released by the U.S. Navy’s 6th Fleet, the Arleigh Burke-class guided-missile destroyer USS Carney has now joined its sister ship, the USS Ross in the waterway that sits on Europe’s eastern flank.

Our decision to have two ships simultaneously operate in the Black Sea is proactive, not reactive,” US Navy Vice Adm. Christopher Grady, the commander of 6th Fleet, said in a press release.

“We operate at the tempo and timing of our choosing in this strategically important region. By nature, ships are flexible, mobile forces, and the Navy is uniquely capable of providing credible and capable forces to defend our nation’s interests throughout the world.”

Saturday’s arrival of the Carney marks the first time two American warships have been in the Black Sea at the same time since Ukraine hosted a training operation in July of last year, but they won’t be alone for long. One day after the Carney’s arrival, the Kremlin announced new naval deployments to the waterway as well, with a Russian frigate, the Admiral Essen, and two patrol ships entering the Black Sea soon thereafter.

Tensions between the NATO alliance and Russia have been on a steady climb since the Russian military annexation of Crimea in 2014. Since then, NATO has been bolstering its defenses in the region, and if their reports of Russian activity in the vicinity are accurate, so has Russia. Because the Kremlin remains tight lipped about the allocation of their defensive assets, there has been no formal confirmation of Russia’s military build up in Crimea and surrounding territory, but the uptick in both safe and unsafe intercepts between Russian and NATO aircraft in recent years seem to support NATO’s claims.

Last year, during massive military exercises Russian conducted in the region along with ally Belarus, many NATO officials voiced concerns about the Russian military leaving staged heavy equipment in the Russian satellite of Kaliningrad, which abuts a notable choke point between NATO allies in the Baltics and the rest of Europe.