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.