KYIV, March 23 - Russia unleashed one of its most devastating attacks against Ukraine’s electric sector on Friday, an aerial assault it said was retaliation for recent strikes inside Russia and which could signal an escalation of the war just days after President Vladimir Putin cemented his grip on power in a preordained election.

Many Ukrainians were plunged into darkness across several cities, at least five people were killed, and damage to the country’s largest hydroelectric plant briefly cut off power to a nuclear plant that has been a safety risk throughout the war.