KIEV, Ukraine — With 24 of his sailors and three of his warships held captive by Russia after a confrontation on Sunday in waters near Crimea, the commander of Ukraine’s tiny navy declared that Russia under President Vladimir V. Putin was “the plague of the 21st century” and vowed that it must be stopped.

But the commander was not suggesting that Ukraine fight to the last man. “Of course, we can’t beat them,” the commander, Vice Adm. Ihor Voronchenko, acknowledged.

Instead, Ukraine has proposed that Turkey, a member of NATO, seal off the Bosporus, one of the world’s busiest waterways, and that the alliance send a convoy into the Sea of Azov.

The chances of any of that happening are near zero, but the Ukrainian commander’s Hail Mary proposal highlights the extent to which the country is dependent on outside help as it struggles to hold its own as a sovereign state in the face of a hostile Kremlin.