Ukrainian refugees should not return until spring to help ease pressure on the energy system after a wave of Russian attacks, the government said.
“The networks will not cope,” said Deputy Prime Minister Iryna Vereshchuk. “You see what Russia is doing.”
“We need to survive the winter,” she added.
Ukrainian President Volodymyr Zelensky said Russian air strikes had destroyed more than a third of the country’s energy sector.
Ms Vereshchuk said that although she would like Ukrainians to return in the spring, it was important to refrain from returning for now because “the situation will only get worse”.
“If it is possible, stay abroad for the time being,” she added.
Ukraine’s economy has suffered badly since the war began. Mr Zelensky has called on the world for help urgently to cover an expected budget deficit of $38bn (£33bn) next year.
The International Monetary Fund (IMF) said Ukraine would need $3bn every month to survive the next year – and $5bn if Moscow’s bombardment intensified.
The deputy mayor of the western city of Lviv, Serhiy Kiral, told the BBC on Saturday that Russia’s strategy was to damage critical infrastructure before the winter, and bring the war to areas beyond the front line.
Russia began attacking Ukraine’s energy networks in retaliation for an attack on Russia’s rail and road bridge to Crimea on 8 October.
Areas targeted by the latest attacks include the Cherkasy region, south-east of the capital Kyiv, and the city of Khmelnytskyi, further west.
On Friday Mr Zelensky accused Russia of planting mines at a hydroelectric dam in the Kherson region of southern Ukraine, which is under the control of Moscow’s forces.
He said that if the Kakhovka hydropower plant was destroyed, hundreds of thousands of people would be in danger of flooding. Russia has denied planning to blow up the dam and said Ukraine was firing missiles at it.
Since the Russian invasion of Ukraine in February, the UN’s refugee agency has recorded about 7.7 million refugees from Ukraine across Europe, including Russia, out of a population of about 44 million.