At least three people have been killed and another 17 injured in the biggest Russian drone attack on Ukraine's Kharkiv, the city's mayor says.

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.