"To return now is again to expose yourself, your children, and all your vulnerable relatives, who may be either sick or with limited mobility.”
Ukrainian refugees are being urged not to return home this winter as the country battles to conserve power.
Russian strikes have been targeting Ukraine's energy infrastructure in recent weeks.
Ukraine's president, Volodymyr Zelenskyy, says 30% of Ukraine's power plants have been destroyed since the war began.
Rolling power cuts -- for around four hours a day -- have been imposed in parts of the country in a bid to conserve energy.
“We need to survive the winter, but, unfortunately, the networks will not survive," said Iryna Vereshchuk, the country's deputy prime minister. "The threat of shelling, cold and hunger remains."
"To return now is again to expose yourself, your children, and all your vulnerable relatives, who may be either sick or with limited mobility,” she added.
The UN's refugee agency says 7.7 million people have left Ukraine since the outbreak of war in February, although some have since returned.
Meanwhile, the International Organization for Migration (IOM) says 6.2 million Ukrainians have been displaced within the country.
Around a sixth are currently in urgent need of help, according to Stephane Dujarric, a UN spokesperson.
"There are now 1 million people in 5,670 locations where displaced people are seeking temporary shelter and support, who need help as winter approaches,” said Dujarric. “The most urgent needs are winter clothes, blankets, portable stoves and solid fuel."