New satellite images show apparent mass graves near Mariupol, where local officials accused Russia of burying up to 9,000 Ukrainian civilians to conceal the slaughter taking place in the ruined port city that’s almost entirely under Russian control.

The images emerged just hours after Russian President Vladimir Putin on Thursday claimed victory in the battle for Mariupol, despite the presence of an estimated 2,000 Ukrainian fighters who were still holed up at a giant steel mill. Putin ordered his troops to seal off the stronghold “so that not even a fly comes through” instead of storming it.

Putin’s decision to blockade the Azovstal steel plant likely indicates a desire to contain Ukrainian resistance in Mariupol and free up Russian forces to be deployed elsewhere in eastern Ukraine, Britain's Defense Ministry said in an assessment Friday.

Satellite image provider Maxar Technologies released the photos, which it said showed more than 200 mass graves in a town where Ukrainian officials say the Russians have been burying Mariupol residents killed in the fighting. The imagery showed long rows of graves stretching away from an existing cemetery in the town of Manhush, outside Mariupol.

Mariupol Mayor Vadym Boychenko accused the Russians of “hiding their military crimes” by taking the bodies of civilians from the city and burying them in Manhush.

The graves could hold as many as 9,000 dead, the Mariupol City Council said Thursday in a post on the Telegram messaging app.

Boychenko labeled Russian actions in the city as “the new Babi Yar,” a reference to the site of multiple Nazi massacres in which nearly 34,000 Ukrainian Jews were killed in 1941.

“The bodies of the dead were being brought by the truckload and actually simply being dumped in mounds,” an aide to Boychenko, Piotr Andryushchenko, said on Telegram.

There was no immediate reaction from the Kremlin. When mass graves and hundreds of dead civilians were discovered in Bucha and other towns around Kyiv after Russian troops retreated three weeks ago, Russian officials denied that their soldiers killed any civilians there and accused Ukraine of staging the atrocities.

READ: From Ukraine to Russia: Boy safer, but not closer to US dad

In a statement, Maxar said a review of previous images indicates that the graves in Manhush were dug in late March and expanded in recent weeks.

After nearly two lethal months of bombardment that largely reduced Mariupol to a smoking ruin, Russian forces appear to control the rest of the strategic southern city, including its vital but now badly damaged port.

But a few thousand Ukrainian troops, by Moscow's estimate, have stubbornly held out for weeks at the steel plant, despite a pummeling from Russian forces and repeated demands for their surrender. About 1,000 civilians were also trapped there, according to Ukrainian officials.

Ukrainian officials have repeatedly accused Russia of launching attacks to block civilian evacuations from Mariupol.

At least two Russian attacks on Thursday hit the city of Zaporizhzhia, a way station for people fleeing Mariupol. No one was wounded, the regional governor said.

Among those who arrived in Zaporizhzhia after fleeing the city were Yuriy and Polina Lulac, who spent nearly two months living in a basement with at least a dozen other people. There was no running water and little food, Yuriy Lulac said.

“What was happening there was so horrible that you can’t describe it,” said the native Russian speaker who used a derogatory word for the Russian troops, saying they were “killing people for nothing.”

“Mariupol is gone. In the courtyards there are just graves and crosses,” Lulac said.

The Red Cross said it had expected to evacuate 1,500 people by bus, but that the Russians allowed only a few dozen to leave and pulled some people off of the buses.

Dmitriy Antipenko said he lived mostly in a basement with his wife and father-in-law amid death and destruction.

“In the courtyard, there was a little cemetery, and we buried seven people there,” Antipenko said, wiping away tears.

Instead of sending troops to finish off the Mariupol defenders inside the steel factory in a potentially bloody frontal assault, Russia apparently intends to maintain the siege and wait for the fighters to surrender when they run out of food or ammunition.

All told, more than 100,000 people were believed trapped with little or no food, water, heat or medicine in Mariupol, which had a prewar population of about 430,000. Over 20,000 people have been killed in the siege, according to Ukrainian authorities.

The city has seized worldwide attention as the scene of some of the worst suffering of the war, including deadly airstrikes on a maternity hospital and a theater.

Boychenko rejected any notion that Mariupol had fallen into Russian hands.

“The city was, is and remains Ukrainian,” he declared. “Today our brave warriors, our heroes, are defending our city.”

The capture of Mariupol would represent the Kremlin’s biggest victory yet of the war in Ukraine. It would help Moscow secure more of the coastline, complete a land bridge between Russia and the Crimean Peninsula, which Russia seized in 2014, and free up more forces to join the larger and potentially more consequential battle now underway for Ukraine’s eastern industrial heartland, the Donbas.

At a joint appearance with Russian Defense Minister Sergei Shoigu, Putin declared, “The completion of combat work to liberate Mariupol is a success,” and he offered congratulations to Shoigu.

Shoigu predicted the Azovstal steel mill could be taken in three to four days. But Putin said that would be “pointless" and expressed concern for the lives of Russian troops in deciding against sending them in to clear out the sprawling plant, where the die-hard defenders were hiding in a maze of underground passageways.

Instead, the Russian leader said, the military should “block off this industrial area so that not even a fly comes through.”

The plant covers 11 square kilometers (4 square miles) and is threaded with some 24 kilometers (15 miles) of tunnels and bunkers.

“The Russian agenda now is not to capture these really difficult places where the Ukrainians can hold out in the urban centers, but to try and capture territory and also to encircle the Ukrainian forces and declare a huge victory,” retired British Rear Adm. Chris Parry said.

Russian officials for weeks have said capturing the mostly Russian-speaking Donbas is the war’s main objective. Moscow’s forces opened the new phase of the fighting this week along a 300-mile (480-kilometer) front from the northeastern city of Kharkiv to the Azov Sea.

While Russia continued heavy air and artillery attacks in those areas, it did not appear to gain any significant ground over the past few days, according to military analysts, who said Moscow's forces were still ramping up the offensive.

Despite Russia’s renewed focus, its troops are still suffering from losses sustained earlier in the conflict, according to the British assessment. In order to try and reconstitute their depleted forces, the Russians have resorted to sending inoperable equipment back to Russia for repair, it said.

A senior U.S. defense official, speaking on condition of anonymity to discuss the Pentagon’s assessment, said the Ukrainians were hindering the Russian effort to push south from Izyum.

In the U.S., President Joe Biden pledged an additional $1.3 billion for new weapons and economic assistance to help Ukraine, and he promised to seek much more from Congress to keep the guns, ammunition and cash flowing.