Russia bombarded Ukraine’s second-largest city with massive strikes in the early hours of Saturday, its mayor said, one night after Moscow carried out one of the war’s largest aerial assaults on Ukraine.

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.

Air defense has been activated in Kyiv amid reports of airstrikes targeting Chernihiv, a city in northern Ukraine, ABC News confirmed.

Russian ballistic missiles have been launched toward Ukraine from multiple directions, the Ukrainian Air Force said on Telegram Friday.

Technological advances have given threadbare militaries and rebel fighters the ability to strike stunning blows against far stronger forces.