Ukraine's military intelligence chief Kyrylo Budanov has revealed how his country sank a Russian missile ship on Thursday off the coast of Crimea.

On Friday, Budanov said maritime drones made six direct hits on the vessel, named Ivanovet. He says it capsized after being struck in the hull.

Ukrainian media say the Ivanovet is one of three missile ships of the same type belonging to Russia's Black Sea Fleet. They describe the sinking as a big loss for Moscow.

Ukraine also reportedly attacked an airfield in Crimea on Wednesday. Air Force spokesperson Yurii Ihnat said on Friday that the operation damaged at least three Russian military aircraft.

Ukraine is apparently stepping up its counteroffensive in and around Crimea, which Russia controls.

Fighting between the two sides is deadlocked in Ukraine's east.

Still, Russian President Vladimir Putin is vowing to continue with the invasion.

Putin spoke on Friday in Tula, a hub for the Russian defense industry.

He said weapons manufacturers are working hard, and annual output has risen several-fold.

Defense Minister Sergei Shoigu told a ministry meeting that Russian troops maintain the strategic upper hand on all battlefronts, and are expanding their areas of control.