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.

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,