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

Russia bombarded six regions of Ukraine in one of its largest aerial attacks of the three-year war, Ukrainian officials said Friday. The nighttime assault lasted for hours and killed three emergency responders in the capital Kyiv,

Operation of the city's metro transport system was disrupted because a Russian strike had hit and damaged a train between stations. The air attack also triggered fires in residential buildings.