Mariupol possesses strategic importance to Russia because it is situated between the Crimean peninsula, which Russia has occupied since 2014, and the breakaway Ukrainian regions in the eastern part of the country, which have pro-Russian separatist factions. As a result, the city has faced some of the most brutal Russian war tactics since the invasion began on Feb. 24.

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Also on Monday, nine volunteers working for a Ukrainian group that provides food and medicine to those in need were detained by Russian forces and remain missing, according to CNN. Last week, an International Committee of the Red Cross team repeatedly but unsuccessfully tried to get to Mariupol to help civilians evacuate, it said in a statement.

The ICRC blamed “security conditions” making it “impossible to enter” the city, though it said “the ICRC remains ready to facilitate the safe passage of civilians from Mariupol.”

Pentagon spokesman John Kirby said the Defense Department is unable to verify Zelensky’s estimate, adding, “I don’t think anybody’s really going to know until Ukrainian authorities are able to get in and look.”

The Russians’ tactics make it “inconceivable to imagine that there aren’t going to be civilian casualties and that it could be a significant number,” he added.

The Mariupol City Council accused Russian forces of using a mobile crematorium to cover up their actions in the city after there was global condemnation for alleged war crimes in the city of Bucha, which is near Kyiv.

Prior to the reporting of the massacre in Bucha, Russia shelled a maternity hospital in Mariupol, bombed a Mariupol theater that had been serving as a shelter — even though it had spelled out the word “children” in Russian in the front and back of the facility — and bombed a school that was housing hundreds of people in the city.

Mariupol Mayor Vadym Boichenko previously estimated the death toll to be about half of Zelensky’s estimation, whereas the United Nations has recorded roughly 1,842 fatalities and nearly 2,500 injuries.

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The U.N. warns that the death toll is likely “considerably higher,” considering the difficulty of wartime casualty tallying, and Mariupol is one of the cities it says likely has a higher death toll than what is known based on the “intense hostilities” in the area.

The Pentagon said last week that Russian forces near Kyiv retreated and will eventually be redeployed to eastern Ukraine, their new focus in the military offensive, though it won’t happen with “great speed,” a senior U.S. defense official told reporters on Friday: “We don’t believe that, in general, this is going to be a speedy process for them given the kinds of casualties they’ve taken and the kind of damage that they’ve sustained to their units’ readiness.”