Ukrainian officials have estimated that about 2,000 of their troops are inside the plant along with the civilians sheltering in the facility's underground tunnels.
The apparent mass grave seen in satellite images covers a space larger than three football fields. The imagery shows rows of graves stretching away from an existing cemetery in Manhush, near Mariupol.
The Russian president called it a victory, but Ukrainian soldiers maintain control of a sprawling steel plant. Putin said a blockade of the plant will save the lives of Russian fighters.
"It is either die or leave. We had no other way," says one woman who arrived in Zaporizhzhia after three attempts at escape. Officials say nearly 100,000 people are still trapped in the besieged city.
More than 70 hospital patients and staff members from the besieged Ukrainian city have been sent to Russia against their will, officials say, a war crime under the Geneva Conventions.