Mariupol's city council claimed that Russian troops were using mobile crematoria in the besieged port city to "cover their tracks."
It quotes eyewitnesses as saying that "special brigades" are collecting and burning "the bodies of Mariupol residents murdered and killed as a result of the Russian invasion".
Claiming that Russia's top military commanders ordered the destruction of evidence, the council says the real number of fatalities could far exceed last week's cautious estimates of 5,000 deaths and run into "tens of thousands of civilians".
The city council said that this explained Russian reluctance to allow in Turkish and other initiatives to evacuate Mariupol, saying the world had not seen the scale of tragedy in Mariupol since World War II.
Ukrainian President Volodymyr Zelenskyy also said in an interview with Turkish media that Russian soldiers were "cleaning up" before allowing aid workers to see what's happening in the city.
Officials have said that more than 90% of the city's infrastructure had been destroyed by Russian shelling.
Capturing the port city would allow Russia to secure a continuous land corridor to the Crimean Peninsula, which Russia seized from Ukraine in 2014.
Ukraine's defence ministry said on Wednesday that Russian forces were continuing to shell the city.