Russia has been pounding Ukraine's energy system since the start of its invasion, in what Kyiv says is an attempt to sap morale and weaken Ukrainians' resistance.
Ukraine's President Volodymyr Zelenskyy said on Wednesday that around 4,000 buildings in the capital Kyiv were still without heating and most of the city was cut off from electricity after Russian strikes earlier in the week.
"As of this morning, about 4,000 buildings in Kyiv are still without heat and nearly 60 percent of the capital is without electricity," Zelenskyy said, after Russia pounded Ukraine's energy grid between Monday and Tuesday.
Russia has repeatedly hit Ukrainian energy infrastructure throughout the nearly four-year war, but Kyiv says this winter has been the toughest yet, with hundreds of Russian drones and missiles overwhelming air defences during particularly fierce frosts.
The situation also remained difficult across the country, including in the Sumy, Chernigiv, Dnipro and Kharkiv regions, Zelenskyy added.
Zelensky announced a "state of emergency" in the country's energy sector last week, with Kyiv schools remaining closed, street lights dimmed and roughly half a million residents leaving the capital.
Journalists for the AFP news agency in Kyiv have seen shops and restaurants closed due to the outages, out-of-order traffic lights and public lighting shut off at night in parts of the capital.
More than 1,000 Ukrainians have been taken to hospital with frostbite and hypothermia over the past 30 days, the health ministry said, adding that the situation could "become threatening if temperatures drop or weather conditions worsen".
Germany called the Russian winter airstrikes against energy infrastructure "war crimes."
Zelenskyy has repeatedly criticised Kyiv city hall, led by Vitali Klitschko, a former boxing champion and political rival, for not doing enough to aid the recovery.
"According to reports from city authorities, the resources involved are sufficient, but time is needed. I do not agree with this assessment –- additional measures and additional resources are needed," Zelenskyy said.
Klitschko said utility workers were working around the clock to restore heating, water and electricity, but that repeated strikes and frosts were making their job harder.
He also said the situation was "complicated because most of these buildings are being connected for the second time after the damage to critical infrastructure on January 9."
Kyiv authorities have set up hundreds of tents across the city, where people can warm up and get hot food and drink.
Russia has been pounding Ukraine's energy system since the start of its invasion, in what Kyiv says is an attempt to sap morale and weaken Ukrainians' resistance.
The Kremlin says it only targets Ukrainian military facilities and has blamed the continuation of the war on Kyiv for refusing to accept its peace demands.
The Russian defence ministry said on Tuesday it had carried out strikes on facilities that support Ukraine's military.
The International Criminal Court has issued arrest warrants for two top Russian military officials over the attacks on Ukraine's energy grid.
The court said it constituted a war crime as it was designed to harm Ukrainian civilians.
Due to war-time sensitivities, Kyiv does not say which energy facilities have been damaged or destroyed in Russian attacks.