Thousands of civilians may have died in the besieged port city of Mariupol since the bombing began four weeks ago.

"We do think that there could be thousands of deaths, of civilian casualties, in Mariupol," Matilda Bogner, head of the U.N. human rights mission in Ukraine which deploys some 60 monitors, said in a virtual interview, Reuters reports.

She said the mission did not have a precise estimate but was working to gather more information.

According to Mariupol Mayor's spokesman Vadym Boychenko, nearly 5,000 people, including about 210 children, have been killed in the town after Russian troops besieged it a month ago. Ninety percent of Mariupol's buildings have been damaged and forty percent have been destroyed, including hospitals, schools, kindergartens, and factories.

Local officials, citing witness accounts, last week estimated that 300 people were killed in the March 16 bombing of a Mariupol theatre where people were sheltering.

"On the mass graves, actually we've decided now we should be calling it 'improvised graves'," Bogner said on Tuesday.

This was because the term "mass graves" may imply victims of a crime, whereas people who have perished in Mariupol reflect deaths from a range of causes, she said.

Civilian casualties from the conflict were believed to be a "fairly small portion" of bodies in the improvised graves in parks and gardens, she said. Some people who died naturally were never taken to morgues or individual tombs due to the hostilities, while others never reached doctors, she added. It was not clear if any military casualties are buried in the improvised graves, Bogner said.

As of Tuesday, the UN Human Rights Office confirmed 1,179 killed and 1,860 wounded civilians across Ukraine since the beginning of Russia’s invasion on February 24.

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