WASHINGTON (Reuters) -The United States believes that Russian President Vladimir Putin's threatened retaliation against Ukraine over its drone attack last weekend has not happened yet in earnest and is likely to be a significant, multi-pronged strike, U.S. officials told Reuters.

Ukraine’s shock drone strike on Russia’s strategic bomber fleet this week has generals and analysts taking a new look at threats to high-value United States aircraft at bases in the homeland and abroad – and the situation is worrisome.

Ukraine unleashed more than a hundred drones smuggled deep into Russia in what it called its most damaging attack yet.