The Kremlin has fired its top naval commander, the biggest fallout yet from a series of devastating attacks by Ukraine on Russia’s Black Sea Fleet, according to a Ukrainian and a Western official.

Adm. Nikolai Yevmenov, the head of the Russian Navy for the past five years, was removed from command and replaced by the head of the Russia’s Northern Fleet.

Russian publications, citing anonymous sources, reported on Sunday that Admiral Yevmenov had been fired. The Financial Times, citing Ukrainian officials, reported the development on Monday. The Russian government, however, has declined to confirm any of the personnel changes.

U.S. officials have assessed that while Kyiv’s counteroffensive last year in eastern and southern Ukraine largely failed, its strikes on the Crimean Peninsula and attacks on the Black Sea Fleet were unexpectedly effective.

The victories have been all the more surprising because Ukraine does not have a traditional navy or a fleet of warships. Instead, Ukraine has used sea drones and missiles to attack Russian ships.

U.S. officials believe Ukraine has sunk 15 Russian ships in the past six months. European officials have said the naval victories have reopened the western Black Sea, allowing Ukraine to again ship grain from Odesa.