Ukraine said on Wednesday that its forces had sunk a large Russian ship off the coast of Crimea before dawn, in what would be another powerful blow against the Russians at sea, as outgunned Ukrainian soldiers struggle to hold back bloody assaults on land.

The Ukrainian military released footage of the strike, which it said had resulted in the sinking of the 360-foot-long landing ship Caesar Kunikov, possibly complicating Russia’s logistical efforts in southern Ukraine.

The Ukrainian claim could not be immediately confirmed, but when NATO’s secretary general, Jens Stoltenberg, was asked about the attack, he called Ukraine’s campaign on the Black Sea a “great achievement.”

“The Ukrainians have been able to inflict heavy losses on the Russian Black Sea Fleet,” he said at a news conference in Brussels. Russia has lost more than a third of its fleet since the war began, according to Ukrainian officials and military analysts.

Russia declined to comment on the attack.

At the same time, however, Ukrainian ground forces find themselves in perhaps their most precarious position since the opening months of the Russian invasion.

“The enemy is now advancing along almost the entire front line, and we have moved from offensive operations to conducting a defensive operation,” Ukraine’s top military commander, Gen. Oleksandr Syrsky, told the German outlet, ZDF, in his first interview since being promoted to the post last week.