The U.N. human rights chief said on Thursday that Russian strikes on critical infrastructure in Ukraine since October had killed at least 77 civilians and were plunging millions of people into extreme hardship.

"Millions are being plunged into extreme hardship and appalling conditions of life by these strikes," said Volker Turk in a statement. "Taken as a whole, this raises serious problems under international humanitarian law, which requires a concrete and direct military advantage for each object attacked."

In the same statement, he said the U.N.'s preliminary analysis of videos that appeared to show Ukrainian soldiers executing Russian prisoners of war indicated they were "highly likely to be authentic". (Reporting by Emma Farge, editing by Rachel More)