Western officials said that the Russian Black Sea fleet had gone into a defensive posture after a series of setbacks. They said that at least half of the Black Sea Fleet's naval aviation arm had been taken out of action in the attack on Saki air base in Crimea last week.
The naval aviation arm of the fleet, the 34th Naval Ground Attack Aviation Regiment, has two squadrons of roughly two dozen combat aircraft altogether (including Su-24M and Su-30SM aircraft), according to published accounts. Previous assessments suggested at least seven — and maybe as many as 10 — aircraft had been damaged or destroyed in last week's attack.
Russia still has dozens of combat planes at two other bases in the Crimea, but they are not specifically assigned to the Black Sea fleet.
The officials said at a briefing on Friday that the Russian "system" was seeking to apportion blame for the losses, which the Russian defense ministry attributed to an accidental explosion of ammunition at the base.
The officials said the Ukrainian side was probably able to bring several assets to bear in Crimea and that Ukrainian forces do have the ability to operate behind Russian lines. Whether Ukrainian attacks in the south would have a strategic effect on the conflict would depend on whether Ukraine can sustain the level of activity, they added.
The officials said that Russian forces were experiencing problems in resupplying troops in the southern Kherson region, and they estimated that somewhere in the "low thousands" of additional Russian troops had been moved to the region in preparation for a possible Ukrainian offensive.
It remained to be seen, they said, how and when that offensive might take shape.
The officials said that the rise in rocket attacks by the Russians against Kharkiv, which have killed more than a dozen people in recent days, did not presage a Russian offensive in the area but a desire to prevent the redeployment of Ukrainian defenses elsewhere.