Russians are using mobile crematoria to conceal their atrocities and destroying traces of its army's crimes, Ukraine claims.
Preliminary estimates suggest at least 5,00 people died after the alleged genocide in Bucha, Kyiv Oblast.
The Centre for Strategic Communications and Information Security under the Ministry of Culture and Information Policy of Ukraine tweeted: "In Mariupol, the Russian occupiers have started using mobile crematoria to conceal their atrocities, the city council says.
"Amid the #BuchaMassacre, the Russian leadership ordered to destroy the traces of its army's crimes in Mariupol.
"The occupiers are trying to identify and eliminate all potential witnesses to the atrocities.
"Local pro-Russian collaborators and "DPR" terrorists are burning the bodies of murdered residents.
"The process is coordinated by the self-proclaimed mayor Konstantin Ivashchenko."
In a statement, Mariupol City Council reported: "One week ago, cautious estimates put the death toll at 5,000.
"But given the size of the city, catastrophic destruction, the duration of the blockade and fierce resistance, tens of thousands of civilians from Mariupol could have fallen victim to the occupiers.
"All the dirty work of the Russians left for collaborators.
"Eyewitnesses report that the racists involved local and DNR terrorists in special brigades."
Mariupol Mayor Vadym Boychenko said: "The world has not seen the scale of the tragedy in Mariupol since the Nazi concentration camps.
"The racists have turned our entire city into a death camp."
Russian artillery fire killed at least two people and wounded five at a humanitarian aid distribution point on Wednesday as Moscow's forces bombarded towns and cities in eastern Ukraine, local officials said.
Authorities in the eastern region of Luhansk urged civilians to evacuate "while it is safe," warning that Russian bombardments could cut off escape routes.
Ukraine says Russian troops that invaded on February 24 are regrouping and preparing for a new offensive in the Donbas area, which includes both the Luhansk and Donetsk regions.
Donetsk Governor Pavlo Kyrylenko shared online photos from the town of Vuhledar, where he said Russian artillery fire had struck a humanitarian aid distribution point.
Satellite images of Bucha have been widely shared and appear to show bodies lying in the street almost two weeks before Russians left the town.
The horrifying pictures appear to show bodies in the exact locations where they were subsequently found by Ukrainian forces when they regained control of the town north of Kyiv.
There is an earlier satellite image available from Maxar on March 11 which appears to show bodies in the same locations, but is less clear than the one on March 19.
Ukrainian President Volodymyr Zelensky on Monday described as "genocide" the killings of civilians in the town of Bucha outside the capital Kyiv reclaimed from Russian forces.
He said: "These are war crimes and it will be recognised by the world as genocide.
"You stand here today and see what happened.
"We know that thousands of people have been killed and tortured with extremities cut off, women raped, children killed. It's genocide."
Ukraine and Western nations have accused Russian troops of war crimes after the discovery of mass graves and apparently executed civilians at Bucha, prompting vows of action at the International Criminal Court.
Russia has denied targeting civilians and denies the alleged massacre.
Moscow instead claims the footage and images show "fake dead bodies" which were staged after its troops left the town on March 30.
Local officials reported fighting in many part of eastern Ukraine and there were also reports of shelling and fighting in the south, where the port city of Mariupol is surrounded and under siege from Russian forces.
Mariupol's capture could enable Russia to entrench a land passage between two separatist, self-proclaimed people's republics in Donbas and the Crimea region which Russia seized and annexed in 2014.
