The World Health Organisation (WHO) has recorded 703 attacks on health infrastructure in Ukraine since the start of the Russian invasion.
"This is a clear breach of international humanitarian law and the rules of war," WHO regional director Hans Kluge said in Kiev on Monday, calling it the biggest attack on health care on European soil since World War II.
Hundreds of thousands of houses and flats, schools and hospitals are without heating, he said.
"Continued attacks on health and energy infrastructure mean hundreds of hospitals and healthcare facilities are no longer fully operational, lacking fuel, water and electricity to meet basic needs," said Kluge.
"Half of Ukraine's energy infrastructure is either damaged or destroyed," the regional director said. "Today, 10 million people, a quarter of the population, are without power."
Forecast temperatures of minus 20 degrees Celcius pose a serious health risk. "Cold weather can kill," said Kluge. "This winter will be life-threatening for millions of people in Ukraine."
There is a great threat of respiratory infections such as Covid-19, against which many Ukrainians are not sufficiently protected.
Furthermore, "many will be forced to turn to alternative heating methods like burning charcoal or wood or using generators fuelled by diesel, or electric heaters. These bring health risks," Kluge explained, "including exposure to toxic substances ... as well as accidental burns and injuries."
"All of this is taking its toll on the mental health of the people of Ukraine," said Kluge. "Some 10 million people are at risk of mental disorders such as acute stress, anxiety, depression, substance use and post traumatic stress disorder."