Zelensky said 14 regions plus the capital Kyiv were without power and Ukraine's electrical grid operator Ukrenergo said scheduled hourly power outages would affect the whole of the country on Wednesday.

Russian forces have targeted Ukraine's energy infrastructure with missiles and drone strikes in the run up to winter, when mean temperatures typically drop to several degrees below 0 degree Celsius, with lows of minus 20 degrees.

Some 6.9 million people are believed displaced internally within Ukraine and Eastern European countries such as Slovakia and Hungary are preparing for an influx in coming months.

"An increase in numbers is being felt, and is expected. It is currently up 15 percent,” said Roman Dohovic, an aid coordinator for the eastern Slovak city of Kosice.

Ukrainian forces have been on the offensive in recent months while Russia is regrouping to defend areas of Ukraine it still occupies, having called up hundreds of thousands of reservists over the past month.

Zelensky said his forces would not yield "a single centimeter” in battles for the eastern Ukrainian region of Donetsk while Russian-installed officials said Ukrainian forces were moving into a southern town with tanks. The focal points of the conflict in the industrial region of Donetsk are around the towns of Bakhmut, Soledar and Avdiivka, which have seen the heaviest fighting since Russian forces invaded Ukraine in late February.

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