Just hours after Ukrainian President Volodymyr Zelensky warned of an impending "massive" escalation by Moscow, the skies over the Ukrainian capital erupted in a flurry of inbound drone and missile activity.At least 20 people were killed and several dozen injured in the overnight multi-wave Russian missile and drone assault that overwhelmed Kiev's air defense umbrella. The sheer scale of the bombardment indicates a significant ramping up of Moscow's retaliation strategy, after Russian territory has in turn suffered many weeks - even months - of significant drone attacks particularly targeting oil refineries and energy infrastructure.The capital's Mayor Vitali Klitschko confirmed that six floors of an apartment building had partially collapsed after a direct hit from a Russian projectile. "Kyiv is under attack from ballistic missiles and UAVs," Klitschko wrote on Telegram in the night hours.via Associated PressOfficials also noted that at least two children were among the injured and three dozen locations across the city had been damaged in the attacks, according to AFP. Klitschko also said it marked the "most massive" attack night on the capital to date. BBC explains:Although previous attacks have killed more people, this latest barrage deployed the largest number of weapons on the capital and hit locations over a very wide area of Kyiv.Several neighbourhoods were evacuated as strikes rocked buildings throughout the city, hours after Ukrainian President Volodymyr Zelensky warned Russia was preparing an attack.The Ukrainian Air Force released a fiery Telegram statement in the wake of the assault: "We express our condolences to all the victims, families who lost their relatives and loved ones in this terrible terrorist attack. We will take revenge!" the statement said.Residents says that such attacks are becoming more intense, cover a wider area of the capital region, and last longer. "The attack on Kyiv lasted more than 11 hours and came in several waves starting with a drone strike on Kyiv's historic quarter, setting off a fire in a hotel in the city center," BBC additionally reports.WATCH: Massive overnight Russian missile and drone attack on Kyiv.