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 said on Tuesday.

Speaking at a Geneva news conference, Florence Bauer, Eastern Europe head at the U.N. 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. It takes a fertility rate of 2.1 children per woman to maintain a stable population.

Ukraine, which had a population of over 50 million when the Soviet Union collapsed in 1991, has, like almost all its Eastern European and Central Asian neighbours, undergone severe population decline. In 2021, the last year before Russia's full-scale invasion it had a population of about 40 million.

Bauer said that a precise accounting for the impact of the war on Ukraine's population would have to wait until after the conflict ended when a full census could finally be carried out.

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.

Much-larger Russia, with a pre-war population of over 140 million, has also seen its already dire demographic situation deteriorate since it invaded Ukraine: it recorded its lowest birth rate since 1999 in the first six months of this year, a level even the Kremlin described as "catastrophic".

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.

"It's difficult to have exact numbers, but estimates range around tens of thousands of casualties," she said.