Ukraine's population has declined by more than 10 million since Russia invaded in February 2022, sparking an exodus and sending birth rates plunging, the United Nations said Tuesday.

The UN Population Fund said there had not been a census, but that there clearly had been a dramatic population decline in war-torn Ukraine.

"The Ukraine population has declined by over 10 million since the beginning of the war," UNFPA's regional director for Eastern Europe and Central Asia Florence Bauer told reporters in Geneva.

She stressed that the decline had been seen "since the beginning of the full-scale invasion", and was due to "a combination of factors".

Already before the war, Ukraine had one of the lowest birth rates in Europe, and like many countries in Eastern Europe, it had seen a declining population, as young people left in search of more opportunities, Bauer said.

But since the war, some 6.7 million people fled the country as refugees while the birth rate fell to just around one child per woman, she said.

"That's one of the lowest in the world," she said, stressing that this was well below the theoretical replacement rate of 2.1 children that each woman on average must have to maintain the population size.

At the same time, she said, there are the "several tens of thousands of casualties (from the war), which of course add to the equation".