A US satellite firm says it has identified a mass burial site containing about 200 graves near Mariupol, a city Russian forces have been trying to wrest control of for weeks.
Maxar said its images showed an expansion of graves that began at the end of March.
Local Ukrainian officials accuse the Russians of burying Mariupol civilians killed by Russian troops there.
Moscow has not yet responded.
Russian troops control most of Mariupol after weeks of bombardment and fighting, but some Ukrainian forces remain in a sprawling steelworks in the city. President Putin has abandoned plans to storm the Azovstal plant and instead told troops to seal it off.
The alleged mass grave is near a village called Manhush about 20km (12 miles) west of Mariupol. Maxar said there were four sections of linear rows about 85m long.
The BBC has not independently verified the images.
Mariupol city council earlier issued its own statement accusing the Russians of burying civilians at the same location.
The council said the Russians were digging trenches and "using dump lorries to bring the bodies" and provided its own aerial image of the site, which it said was "already twice as large as the nearby cemetery".
The city's mayor Vadym Boichenko says tens of thousands of civilians may have been killed in Mariupol
Moscow has repeatedly denied accusations by Ukraine and its Western allies that Russian troops and top Kremlin politicians are responsible for mass killings of civilians.
Earlier this month, Maxar images of the town of Bucha outside Kyiv appeared to show bodies of civilians lying in the street nearly two weeks before the Russians left the town as part of a withdrawal from northern Ukraine.
The image from 19 March, first reported by the New York Times and confirmed by the BBC, contradicted Russian Foreign Minister Sergei Lavrov's claim that footage of bodies in Bucha had been "staged" after the Russians withdrew.
Bucha Mayor Anatoliy Fedoruk has said that least 300 civilians were killed in the town.