More than 10,000 people have been murdered in Ukraine since Russia invaded in February 2022, with over half of recent killings occurring deep behind the front lines, according to the United Nations Human Rights Office on Tuesday.

The United Nations human rights mission in Ukraine, which has scores of monitors on the ground, said it expects the true toll to be “significantly higher” than the official number because corroboration work is ongoing.

This includes events in the first months after the invasion, such as the battle for control of Mariupol, where residents reported high civilian casualties.

“Ten thousand civilian deaths is a grim milestone for Ukraine,” said Danielle Bell, who heads the monitoring mission.

“The Russian Federation’s war against Ukraine, now entering into its 21st month, risks evolving into a protracted conflict, with the severe human cost being painful to fathom,” she said.

The vast majority of the deaths have been caused by explosive weapons with a wide-area impact such as shells, missiles and cluster munitions, the United Nations said.

Close to half of the deaths in the last three months have occurred far beyond the front lines, the U.N. said, attributing this to Russian forces’ use of long-range missiles and the late explosion of abandoned ordinances.

Older people who may be unable or unwilling to relocate to safer places make up a disproportionate fraction of those killed in Ukraine, the U.N. data showed.

More than a third of the victims whose ages were confirmed were over 60, it showed, whereas this age group constitutes just a quarter of the total population.

Moscow denies deliberately targeting civilians.