The head of the Norwegian Refugee Council (NRC) expects another wave of hundreds of thousands of refugees from Ukraine in Europe over the winter because of "unlivable" conditions, he said on Monday.

Russia's attacks on Ukraine's electricity infrastructure have left millions of people without heat, clean water or electricity as temperatures plummet.

Moscow says the assaults do not target civilians and are meant to reduce Ukraine's ability to fight and push it to negotiate. Kyiv says the attacks are a war crime.

"Nobody knows how many, but there will be hundreds of thousands more (leaving Ukraine) as the horrific and unlawful bombing of civilian infrastructure makes life unlivable in too many places," Jan Egeland told Reuters by phone after returning from a trip to Ukraine earlier this month.