Ukrainian officials have warned that they are expecting to face a new wave of Russian missile strikes in the next few days.
"The coming week could be as difficult as last week," President Volodymyr Zelensky said in his nightly video address on Sunday.
Previous missile strikes on Ukrainian territory have targeted the country's energy infrastructure, causing massive water and power cuts.
"Our defence forces are preparing, the whole state is preparing," Zelenskyy said.
"We are working out all the scenarios, including with Western partners," he added, calling on Ukrainians to pay attention to anti-aircraft warnings.
Millions of people -- including most of the capital Kyiv -- were left without power last week as winter temperatures set in.
According to the national operator Ukrenergo, around 27% of households are still facing electricity shortages while emergency power cuts are still in force throughout the country.
Ukraine's military says a Russian warship carrying "eight Kalibr-type missiles" has now appeared in the Black Sea.
"It is highly likely that the beginning of the week will be marked by such an attack," said Natalia Goumeniouk, a spokeswoman for the Ukrainian army's southern command.
"This indicates that preparations are underway," she told Ukrainian television on Monday.
The Kremlin last week denied that its attacks on Ukraine's electricity network were aimed at civilians, but said Kyiv could "end the suffering" of its population by meeting Russia's demands to resolve the conflict.
The mayor of Kyiv, Vitali Klitschko, has traded barbs with Ukraine's president over how to help residents withstand power cuts.
Zelenskyy had noted last week that many in the capital had complained about the city's measures following Russia's
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