The very same UN stresses that these numbers severely undercount due to lack of access to occupied territories and mostly reflect deaths in free Ukraine. The figures from the areas where most of the fighting has taken place remain unknown. Realistic estimates go far beyond the death toll in Gaza; people illegally conscripted from the occupied territories into the Russian armed forces alone add several tens of thousands more deaths.

On 11 April, Mariupol Mayor Vadym Boychenko stated that over 10,000 civilians had died in the Russian siege of Mariupol.[323] On 12 April, city officials reported that up to 20,000 civilians had been killed.[323] (this is 1 month into siege) On the same day, the Mayor of the city reported that about 21,000 civilians had been killed.[324] An updated Ukrainian death toll the following month put the number of civilians killed at at least 22,000.[325]

On August 29, President of Mariupol Television, volunteer and civil activist Mykola Osychenko said to Dnipro TV that, according to the insider information, 87,000 deaths have been currently documented in morgues in Mariupol. Besides, 26,750 bodies are buried in mass graves, and many more are buried in the yards of the apartment blocks and private houses, or still under the rubble.[326]

In early November, Ukraine stated that at least 25,000 civilians had been killed in Mariupol.[46][47] In late December, based on the discovery of 10,300 new mass graves, the Associated Press estimated that the true death toll may be up to three times that figure.[327] The Uppsala Conflict Data Program estimates of the total death toll resulting from the siege range from 27,000 to 88,000 fatalities, most of them civilians.[49]

just to put things into perspective, this siege lasted less than 3 months

https://en.wikipedia.org/wiki/Siege_of_Mariupol