The attack on Friday, which killed at least four people nationwide, came two days after Russia vowed retaliation for an audacious drone assault on its strategic bomber bases.

Russia launched large-scale drone and missile strikes on Ukraine's capital and other parts of the country early on Friday, local officials said. At least four people were killed in Kyiv and at least 20 have been injured, the city's mayor Vitaliy Klitschko said.

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.