Russia launched a massive missile attack, backed by drones, on Ukraine’s energy infrastructure on Thursday, leaving millions of people without power, Ukrainian officials said.

The total extent of the damage was not immediately clear, but explosions were heard in cities across the country, and many officials reported power outages.

“The energy sector is under massive enemy attack again,” Ukraine’s energy minister, Herman Halushchenko, wrote on his Facebook page. The operator of the Ukrainian transmission system “has urgently introduced emergency power outages,” Mr. Halushchenko wrote.

In the western regions of Lviv, Volyn and Rivne, more than a million people were without power, officials said. Further to the east, the city of Zhytomyr in central Ukraine was without power and water. The southern city of Kherson was also without power, as was much of Kyiv, the capital.

Ihor Polishchuk, the mayor of Lutsk in the Volyn region of western Ukraine, said there had been several hits on his city and the surrounding area. Six people were reported injured across Ukraine, according to the regional authorities.

Russia has attacked cities in Ukraine with drones almost every night since September in a campaign that analysts say is intended to test and wear down air defenses, and it has recently stepped up missile attacks.