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. The timing of the full Russian response was unclear, with one source saying it was expected within days. A second U.S. official said the retaliation was likely to include different kinds of air capabilities, including missiles and drones.