A grim discovery has been made in a village some 20km (12 miles) outside the city.
US satellite firm Maxar says it has identified a mass burial site containing about 200 graves in Manhush.
Local Ukrainian officials accuse the Russians of burying Mariupol civilians killed by Russian troops there, but Moscow has not yet responded to the allegations.
The city’s mayor Vadym Boichenko says tens of thousands of civilians may have been killed in Mariupol.
The situation on the ground in Mariupol remains extremely difficult, with no further evacuations of civilians on Thursday.
Meanwhile, Russian President Vladimir Putin has ordered his troops to seal off Ukrainian defenders inside the besieged port city of Mariupol.
Mr Putin told forces to abandon plans to storm the sprawling Azovstal steelworks there, where Ukraine is still resisting the invaders.
The Azovstal Iron and Steel Works – a massive, four sq-mile (10 sq km) plant in the southeast of the city – has become the last centre of Ukrainian resistance in Mariupol. Civilians, as well as fighters, are inside the plant.
Russian forces have been trying for weeks to dislodge them.
The BBC’s Toby Luckhurst in Lviv, western Ukraine, has been reporting on Mr Putin’s apparent change of mind, and Ukrainian defiance.
While Russia talks of isolating the Ukrainian defenders in Azovstal, they remain in contact with the outside world, describing conditions inside the huge steelworks.
Captain Svyatoslav Palamar, deputy commander of the Azov Battalion in the complex, told the BBC military that civilians were hiding in several basements in the complex, some of which he and his comrades were unable to reach.
“We know that there are small children there as young as three-months-old,” he said.
In recent days, Russian forces had used artillery and heavy bunker-busting bombs to try to destroy them, but all the attacks had failed, Capt Palamar said.
He added that more than 500 heavily wounded fighters needed medical help, and the bodies of the dead remained unburied.