The UN human rights chief recalled on Wednesday how he took cover in an underground shelter as at least 70 missiles were launched from Russia targeting essential infrastructure in Ukraine.
"I fear that there is one long, bleak winter ahead for Ukraine," Volker Turk told reporters in Kyiv before leaving for Geneva.
Turk said he had spent four days in sub-zero temperatures.
"I have seen for myself the horrors, suffering, and the daily toll that this war against Ukraine by Russia has had on the people of this country," he said.
He said he spoke to the families of prisoners of war who are anxiously waiting for news from their loved ones.
"I've listened to the pain of those whose children are on the frontline; heard about the particular plight of people with disabilities and older people who are unable to reach safe shelter when the air raid sirens go off," said Turk.
Buried under rubble
He said he visited the site of a shelled apartment block in Izium, Kharkiv Oblast, where more than 50 people were buried under the rubble.
"And in the rubble were the signs of lives cut short by acts of indiscriminate shelling. A shoe. A piano. Toys. A closet full of clothes. Shelves of books."
Turk said he had to take shelter underground on Monday as Russian missiles hit civilian targets in Ukraine.
"A group of human rights defenders joined us, and we calmly carried on with our meeting. As if it was normal to have your day interrupted by air raid sirens," said Turk.
On his visit, he witnessed the scale of civilian casualties and the damage caused to civilian infrastructure, including hospitals and schools.
During winter, attacks on civilian sites have "horrible consequences" for the most vulnerable, who struggle with blackouts, with no heating or electricity.
"I talked to a woman who showed me her destroyed apartment block, where all her neighbors, now dead, used to live," said Turk.
Some 17.7 million people in Ukraine need humanitarian assistance, and 9.3 million require food and livelihood assistance.
According to Turk, a third of the population has been forced to flee their homes.
Some 7.89 million have fled the country, mostly women and children, and 6.5 million people are internally displaced.
Turk said the Human Rights Office has worked in Ukraine uninterrupted since 2014 when Russia annexed Crimea – documenting and reporting, working with civil society, and advising the government on compliance with international law and how to respond.