Although Russian warships have made themselves scarce, Russian planes are still flying over the sea. Russian forces frequently bomb Zmiiniy Island and attack cities and towns on the Black Sea coast of Ukraine with drones.

There is also the danger that Russia may lay mines to block sea routes, British intelligence said on Wednesday.

But for the moment, the situation on the Black Sea is a huge embarrassment for the Kremlin, as its second-largest naval force has been humbled by a country with almost no navy.

The Black Sea fleet has been a bone contention between Ukraine and Russia since the collapse of the Soviet Union. Moscow had a special arrangement with Kyiv to keep basing the fleet in Sevastopol, and concern over those basing rights was one of the reasons Russian President Vladimir Putin gave for his illegal annexation of Crimea in 2014.

The challenge to the fleet also endangers Russia’s hold on Crimea, said Volodymyr Zablotskiy, a Ukrainian military and naval expert.

“Without Crimea, this expansion fleet will not be viable, and the capabilities of the Kremlin and the region will be limited. These are the strategic consequences of our future de-occupation of the peninsula,” he said. “It is the fleet that enables the logistics of the Russian forces in this direction. And the key to it is the possession of Sevastopol.”