Authorities in Crimea, which is dependent on Ukraine for electricity, said they had managed to partially reconnect the cities of Simferopol, Yalta and Saki using generators after two pylons were brought down.
The attack has raised concerns the Black Sea peninsula, which was annexed by Russia last year, will not have enough energy after the other two main power lines supplying the region were blown up on Friday.
"By decision of the head of the Crimean republic, a state of emergency has been introduced on the peninsula," the ministry said, adding that hospitals and other important buildings were being supplied by generators.
The first deputy premier of Crimea, Mikhail Sheremet, said the peninsula could only supply half its power needs at most using diesel generators and renewable sources such as wind and solar power, the TASS state news agency reported.
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The incident came after Ukraine's UNIAN agency reported that two of the main power lines into Crimea had been attacked overnight Thursday to Friday.
Ukraine's state energy company Ukrenergo posted pictures of a downed pylon and one with a hole blown through it taken on Friday morning, saying the power lines were brought down at around 4:00 am.
"The nature of the damage shows that it took place as a result of shelling or the use of explosive devices," Ukrenergo said in a statement.