Alarming satellite images: they locate mass graves near Mariúpol 0:51
(CNN) --
Ukrainian authorities say they have identified mass graves outside the city of Mariupol, which they say adds to mounting evidence of Russian war crimes against Ukrainian civilians.
The claim is backed up by photos collected and analyzed by US satellite imaging company Maxar Technologies that appear to show more than 200 new graves at a site on the northwestern edge of Manhush, a town about 12 miles west of Mariupol.
Some 100,000 people remain trapped in Mariupol, which has been under constant bombardment since it was surrounded by Russian forces on March 1, according to Ukrainian officials.
Ukrainian authorities claim that more than 20,000 people in the city have been killed during the assault.
In a Thursday post on the Telegram messaging app, Petro Andriushchenko, an adviser to the mayor of Mariupol, said Russian trucks had picked up bodies from the port city, before "dumping" them in Manhush.
"This is direct evidence of war crimes and attempts to cover them up."
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A satellite image from April 3 shows what appears to be a freshly dug mass grave next to the cemetery.
A satellite image from March 23 shows a graveyard on the edge of Manhush.
A Maxar review of satellite imagery from mid-March to mid-April indicated that the tomb's expansion began between March 23-26 and continued through April.
According to Maxar's analysis, there are more than 200 freshly excavated graves at a site in the extreme northwest of Manhush.
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"According to recent media reports, Russian soldiers have been bringing the bodies of people killed in Mariupol to this location," Maxar said in his analysis.
CNN cannot independently verify claims that the Russians have disposed of bodies in mass graves there.
No firm death toll is available after weeks of heavy shelling of Mariupol.
However, journalists in Mariupol have documented the hasty burial of civilians in the besieged city, and images have surfaced on social media showing bodies apparently left for collection in the city.
Vadym Boichenko, the mayor of Mariupol, said on Thursday that women, children and the elderly had died on the city's streets.
"Sadly, we have seen that the bodies of dead Mariupol residents have started to disappear from the streets of our city," he said.
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According to Boichenko, the mass graves are outside a bypass, near a cemetery.
He said there was a field near the cemetery with ditches 30 meters long.
"And there they bury them, they bring the bodies of the dead in trucks and throw them into these ditches," he said.
Putin proclaims the liberation of Mariupol
Evidence of mass graves on the outskirts of Mariupol emerged as Russian President Vladimir Putin proclaimed the "liberation" of the southeastern port city by Russian forces, even as he called off an attempt to storm the Azovstal steel plant, the last bastion of the Ukrainian defenders inside the city, where civilians have also taken refuge.
Mariupol has been under almost constant attack since the first days of Putin's invasion, with much of the city destroyed by Russian bombing.
Civilian structures targeted included a maternity hospital and a theater where up to 1,300 people were seeking shelter.
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Although many have fled, an estimated 100,000 people still remain in Mariupol and its immediate surroundings, which are reported to be largely under Russian control.
Ukrainian officials, who say the city remains in dispute, warned of a major humanitarian emergency in Mariupol as food and water ran out, electricity and gas cut off, but several attempts to set up evacuation corridors to allowing civilians to escape failed.
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"Unfortunately, today it is not possible to evacuate civilians from Azovstal," Boichenko, the city's mayor, said on Thursday.
"Because we are asking for a stable ceasefire. Somewhere we need a day to be able to accommodate those residents who have been hiding there for 57 days straight, and they are being bombed and bombed and bombed."
US Under Secretary of State for Political Affairs Victoria Nuland said Wednesday that "there was some hope that the Russians might allow" the safe passage of wounded civilians and soldiers from Mariupol.
However, she warned that such an arrangement "has collapsed several times before" and it is ultimately up to the Russians to allow safe passage.
He also added that the Mariupol site speaks "of the brutality of this war" and the war crimes that Vladimir Putin is committing.
Iryna Venediktova, Ukraine's prosecutor general, said earlier this month that her office is investigating 5,800 cases of suspected Russian war crimes, with "more and more" proceedings being opened every day.
Meanwhile, US President Joe Biden has called the atrocities being uncovered in Ukraine a "genocide."
Russia has denied the war crimes charges and says its forces do not target civilians.
War in Ukraine Mariupol