(CNN) Ukrainian officials say they have identified mass graves outside the city of Mariupol, which they say adds to mounting proof of Russian war crimes against Ukrainian civilians.
The claim is supported by photos collected and analyzed by US satellite imagery company Maxar Technologies that appears to show more than 200 new graves at a site on the northwestern edge of Manhush, a town around 12 miles (19 kilometers) to the west of Mariupol.
An estimated 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 officials claim that more than 20,000 people in the city have died during the assault.
In a post Thursday on messaging app Telegram, Petro Andriushchenko, an adviser to the mayor of Mariupol, said Russian trucks had collected bodies from the port city, before "dumping them" in Manhush. "This is direct evidence of war crimes and attempts to cover them up."
A Maxar review of the satellite images from mid-March through to mid-April indicated the expansion of the grave site began between March 23 to 26, and continued into April. According to Maxar's analysis, there are more than 200 newly dug graves at a site on the northwestern edge of Manhush.
"According to recent media reports, Russian soldiers have been taking the bodies of people killed in Mariupol to this location," Maxar said in its analysis.
CNN cannot independently verify claims Russians have disposed of bodies in mass graves at that location. A firm death toll following weeks of heavy bombardment of Mariupol is not available.
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 Thursday that women, children and elderly had died on the streets of the city.
"Unfortunately, we have seen that the bodies of dead Mariupol residents have begun to disappear from the streets of our city," he said.
According to Boichenko, the mass graves are off a bypass road, near a cemetery. He said there was a field near the cemetery with 30 meter-long (98-feet-long) ditches.
"And there they bury them, bring the bodies of the dead by trucks and throw them into these ditches," he said.
Evidence of mass graves outside Mariupol surfaced 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 final bastion of Ukrainian defenders inside the city, where civilians have also sheltered.
Mariupol has been under near constant attack since the early day's of Putin's invasion, and much of the city has been destroyed by Russian shelling. Civilian structures targeted included a maternity hospital and a theater where up to 1,300 people were seeking refuge.
Though 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 maintain the city remains contested, have warned of a major humanitarian emergency in Mariupol as food and water run out, with electricity and gas cut -- but several attempts to establish evacuation corridors to allow civilians to escape have failed.
"Unfortunately, it is not possible today to evacuate civilians from Azovstal," Boichenko, the city's mayor, said Thursday. "Because we are asking for a stable ceasefire. Somewhere we need one day to be able to accommodate those residents who have been hiding there for 57 days in a row, and they are being bombed, bombed and bombed."
US Undersecretary of State for Political Affairs Victoria Nuland said Wednesday that there "was some hope that the Russians might allow" safe passage for civilians and wounded soldiers from Mariupol. However, she warned that such an arrangement "has fallen apart a number of times before" and ultimately it is up to the Russians to allow safe passage.
She also added that the siege of Mariupol speaks "to the brutality of this war" and the war crimes Vladimir Putin is committing.
Iryna Venediktova, Ukraine's prosecutor general, said earlier this month that her office is investigating 5,800 cases of alleged Russian war crimes, with "more and more" proceedings opening every day.
US President Joe Biden, meanwhile, has called the atrocities being uncovered in Ukraine a "genocide."
Russia has denied allegations of war crimes and claims its forces do not target civilians.