Ukraine's population has declined by 10 million, or around a quarter, since the start of Russia's full-scale invasion as a result of refugees leaving, collapsing fertility and war deaths, the United Nations (UN) said on Tuesday, Reuters reports.

Speaking at a Geneva news conference, Florence Bauer, Eastern Europe head at the UN Population Fund, said the invasion in February 2022 had turned an already difficult demographic situation into something more severe.

"The birth rate plummeted and is currently at around one child per woman, which is one of the lowest in the world," she said.

The immediate impact was on regions that were all-but depopulated, villages with only old people left, and couples unable to start families, she said.

The largest chunk of Ukraine's population decline was accounted for by the 6.7 million refugees now living abroad, primarily in Europe.

War deaths were also a factor.