Ukraine braced for a climactic battle for control of the besieged country's industrial east after Russian forces withdrew from the shattered outskirts of Kyiv to regroup and intensify their offensive across the Donbas region, where authorities urged people to evacuate before time runs out.

The mayor of the southern port city of Mariupol said Wednesday that more than 5,000 civilians had been killed there. Meanwhile, in areas north of the capital, Ukrainian officials gathered evidence of Russian atrocities amid signs Moscow’s troops killed people indiscriminately before retreating over the past several days.

In his nightly address, Ukrainian President Volodymyr Zelenskyy warned that the Russian military is building up its forces for a new offensive in the east, where the Kremlin has said its goal is to “liberate” the Donbas, Ukraine’s mostly Russian-speaking industrial heartland. Ukraine, too, was preparing for battle, he said.

“We will fight and we will not retreat,” he said. “We will seek all possible options to defend ourselves until Russia begins to seriously seek peace. This is our land. This is our future. And we won’t give them up.”

Ukrainian authorities urged people living in the Donbas to evacuate immediately.

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“Later, people will come under fire,” Deputy Prime Minister Iryna Vereshchuk said, “and we won’t be able to do anything to help them.”

A U.S. defense official speaking on condition of anonymity said Russia had completed pulling out all of its estimated 24,000 or more troops from the Kyiv and Chernihiv areas in the north, sending them into Belarus or Russia to resupply and reorganize, probably to return to fight in the east.

But a Western official, also speaking on condition of anonymity to discuss intelligence estimates, said it will take Russia’s battle-damaged forces as much as a month to regroup for a major push on eastern Ukraine.

Meanwhile, the U.S. and its Western allies have moved to impose new sanctions against the Kremlin over killings they labeled as war crimes.

Mariupol Mayor Vadym Boichenko said that of the more than 5,000 civilians killed during weeks of Russian bombardment and street fighting, 210 were children. Russian forces bombed hospitals, including one where 50 people burned to death, he said.

Boichenko said more than 90% of the city’s infrastructure was destroyed. The attacks on the strategic city on the Sea of Azov have cut off food, water, fuel and medicine and pulverized homes and businesses.

British defense officials said 160,000 people remained trapped in the city, which had a prewar population of 430,000. A humanitarian relief convoy accompanied by the Red Cross has been trying for days without success to get into the city.

Capturing Mariupol would allow Russia to secure a continuous land corridor to the Crimean Peninsula, which Moscow seized from Ukraine in 2014.

In the north, Ukrainian authorities said the bodies of least 410 civilians have been found in towns around Kyiv, victims of what Zelenskyy has portrayed as a Russian campaign of murder, rape, dismemberment and torture. Some victims had apparently been shot at close range. Some were found with their hands bound.

In his address Wednesday night, Zelenskyy accused Russia of interfering with an international investigation into possible war crimes by removing corpses and trying to hide other evidence in Bucha, northeast of Kyiv.

“We have information that the Russian troops have changed tactics and are trying to remove the dead people, the dead Ukrainians, from the streets and cellars of territory they occupied,” he said. “This is only an attempt to hide the evidence and nothing more.”

Switching from Ukrainian to Russian, Zelenskyy urged ordinary Russians “to somehow confront the Russian repressive machine” instead of being “equated with the Nazis for the rest of your life.”

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He called on Russians to demand an end to the war, “if you have even a little shame about what the Russian military is doing in Ukraine.”

In reaction to the alleged atrocities outside Kyiv, the U.S. announced sanctions against Putin's two adult daughters and said it is toughening penalties against Russian banks. Britain banned investment in Russia and pledged to end its dependence on Russian coal and oil by the end of the year.

The U.S. Senate planned to take up legislation Thursday to end normal trade relations with Russia and to codify President Joe Biden’s executive action banning imports of Russian oil. The trade suspension would allow Biden to enact higher tariffs on certain Russian imports.

The European Union is also expected to take additional punitive measures, including an embargo on coal.

The Kremlin has insisted its troops have committed no war crimes and alleged the images out of Bucha were staged by the Ukrainians.

More bodies were yet to be collected in Bucha. The Associated Press saw two in a house in a silent neighborhood. From time to time there was the muffled boom of workers clearing the town of mines and other unexploded ordnance.

Workers at a cemetery began to load more than 60 bodies into a grocery shipping truck for transport to a facility for further investigation.

Police said they found at least 20 bodies in the Makariv area west of Kyiv. In the village of Andriivka, residents said the Russians arrived in early March and took locals’ phones. Some people were detained, then released. Others met unknown fates. Some described sheltering for weeks in cellars normally used for storing vegetables.

“First we were scared, now we are hysterical,” said Valentyna Klymenko, 64. She said she, her husband and two neighbors weathered the siege by sleeping on stacks of potatoes covered with a mattress and blankets. “We didn’t cry at first. Now we are crying.”

To the north of the village, in the town of Borodyanka, rescue workers searched through the rubble of apartment blocks, looking for bodies.

Thwarted in their efforts to swiftly take the capital, increasing numbers of Putin’s troops, along with mercenaries, have been reported moving into the Donbas.

Ukrainian forces have been fighting Russia-backed separatists in the Donbas since 2014. Ahead of its Feb. 24 invasion, Moscow recognized the Luhansk and Donetsk regions as independent states.

The United States and the United Kingdom boycotted an informal meeting Wednesday of the U.N. Security Council called by Russia to press its baseless claims that the U.S. has biological warfare laboratories in Ukraine. The meeting was the latest of several moves by Russia that have led Western countries to accuse Moscow of using the U.N. as a platform for disinformation to divert attention from the war.

Russia's allegations have previously been debunked. Ukraine does own and operate a network of biological labs that have received funding and research support from the U.S. and are not a secret. The labs are part of a program that aims to reduce the likelihood of deadly outbreaks, whether natural or manufactured.

The U.S. efforts date to work in the 1990s to dismantle the former Soviet Union’s program for weapons of mass destruction.