Multiple Ukrainian cities including Kherson have gradually restored power, aided by the reconnection of the country's four nuclear plants, though about six million Ukrainians are still without power.

President Volodymyr Zelenskyy pleaded with Ukrainians to use energy sparingly. "If there is electricity, this doesn't mean you can turn on several powerful electrical appliances at once," he said in an evening video address.

Ukrainian national power grid operator Ukrenergo said several hours earlier that 30 percent of electricity supplies were still out, and asked people to cut back on their energy use. "Repair crews are working around the clock," it said in a statement on Telegram.

Ukrainian authorities have set up 3,720 emergency shelters across the country amid an energy supply cutoff due to the ongoing Russia-Ukraine conflict.

The centers, equipped with power generators, will provide people with heat, electricity, water, internet and medicines.

Kyiv said Russia has incessantly shelled Kherson, the southern Ukrainian city that it retreated from earlier this month. The head of the local administration said on Friday that 15 people had been killed and 35 wounded in the last six days.

Russia insisted it does not target civilians in the "special military operation" it launched in late February.

The International Atomic Energy Agency said the three nuclear plants on Ukrainian-held territory had been reconnected to the grid, two days after the ongoing conflict forced them to shut for the first time in 40 years.

The fourth station, in Zaporizhzhia, is in Russian-controlled territory. It came back online on Thursday.

The European Union will step up efforts to provide Ukraine with support to restore and maintain power and heating, the head of the European Commission said.

UN human rights chief Volker Turk said in a statement that "Millions are being plunged into extreme hardship and appalling conditions of life."

(With input from agencies)