The expected U.S. provision of Patriot missiles to Ukraine is a muscular message of support and will boost defenses against Russia's relentless aerial assaults, but experts say its battlefield impact will be limited.

Patriots are "far from a silver bullet," against the low-flying cruise missiles and drone bombs that Russian forces have pummeled Ukraine with, according to Ian Williams of the Missile Defense Project at the Center for Strategic and International Studies in Washington.

But they will add a layer of protection on top of Ukraine's current systems, and also defend against short-range ballistic missiles that Western officials think Russia is seeking from Iran, Williams said.