Six weeks of brutal Russian siege have left more than 10,000 civilians dead in the southern port city of Mariupol and corpses “carpeted through the streets,” the mayor of that cut-off city said, as the West warned that a Russian convoy and other troops and weapons were on the move for a suspected planned Russian assault in Ukraine’s east.

Mariupol has been the site of some of the heaviest attacks and civilian suffering in the 6-week-old war, but the land, sea and air assaults by Russian forces fighting to capture it have increasingly limited information on circumstances inside the city.

Speaking by phone Monday with The Associated Press, Mariupol Mayor Vadym Boychenko accused Russian forces of having blocked weeks of attempted humanitarian convoys into the city in part to conceal the carnage there from the outside world. Boychenko said the death toll there could surpass 20,000.

Boychenko also gave new details of allegations by Ukrainian officials in recent weeks that Russian forces have brought mobile cremation equipment to Mariupol to dispose of the corpses of victims of the siege.

Russian forces have taken many bodies to a huge shopping center where there are storage facilities and refrigerators, Boychenko said.

“Mobile crematoriums have arrived in the form of trucks: You open it, and there is a pipe inside and these bodies are burned,” he said.

Boychenko spoke from a location in Ukrainian-controlled territory but outside Mariupol. The mayor said he had several sources for his description of the alleged methodical burning of bodies by Russian forces in the city, but did not further detail the sources of his information.

The discovery of large numbers of apparently executed civilians after Russian forces retreated from cities and towns around the capital, Kyiv, this month already has prompted widespread condemnation and charges from Ukrainians and from Western leaders that Russia is committing war crimes in Ukraine.

Elsewhere, U.S. officials point to new signs that Russia’s military is gearing up for a major offensive in Ukraine’s eastern Donbas region, switching its focus after Russian forces failed in their initial drive to capture Kyiv.

Donbas has been torn by fighting between Russian-allied separatists and Ukrainian forces since 2014, and separatists there have declared independent states. Military strategists say Russian leaders appear to hope for more local support and logistics and terrain in Donbas that favor Russia’s larger and better-armed military, potentially allowing Russian troops to gain more territory and weaken Ukraine’s fighting forces.

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Russia has appointed a seasoned general to lead its renewed push in the eastern Donbas region.

A senior U.S. defense official on Monday described a long Russian convoy now rolling toward the eastern city of Izyum with artillery, aviation and infantry support, as part of redeployment for what appears to be the looming Russian campaign.

More artillery is being deployed near the city of Donetsk, while ground combat units that withdrew from around the Kyiv and Chernihiv areas appear destined for refitting and resupplying before they position in Donbas, said the official, who spoke on condition of anonymity to discuss internal U.S. military assessments.

With their offensive in many parts of the country thwarted, Russian forces have relied increasingly on bombarding cities — a strategy that has flattened many urban areas and killed thousands of people.

The U.N. children’s agency said nearly two-thirds of all Ukrainian children have fled their homes in the six weeks since Russia’s invasion began. The United Nations has verified that 142 children have been killed and 229 injured, though the actual numbers are likely much higher.

Ukrainian authorities accuse Russian forces of committing atrocities, including a massacre in the town of Bucha, outside Kyiv, airstrikes on hospitals and a missile attack that killed at least 57 people last week at a train station.

In Bucha, the work of exhuming bodies from a mass grave in a churchyard resumed.

Galyna Feoktistova waited for hours in the cold and rain in hopes of identifying her 50-year-old son, who was shot and killed more than a month ago, but eventually she went home for some warmth. “He’s still there,” her surviving son, Andriy, said.

In Mariupol, about 120,000 civilians are in dire need of food, water, warmth and communications, the mayor said.

Only those residents who have passed the Russian “filtration camps” are released from the city, Boychenko said.

Ukrainian officials say Russian troops are confiscating passports from Ukrainian citizens, then moving them to “filtration camps” in Ukraine’s separatist-controlled east before sending them to distant, economically depressed areas in Russia.

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Boychenko said Monday that those who did not pass the “filtering” have been moved to improvised prisons. He put the number of people taken to Russia or separatist territory in Ukraine at 33,000 or more.

Russian has denied moving people against their will.

President Volodymyr Zelenskyy warned Ukrainians that Russia might use chemical weapons in Mariupol. “We take this as seriously as possible,” Zelenskyy said in his nightly address Monday.

Western leaders warned even before Russian troops moved into Ukraine that Russia could resort to unconventional weapons there, particularly chemical agents.

A Russia-allied separatist official, Eduard Basurin, appeared to urge their use Monday, telling Russian state TV that Russian-backed forces should seize a giant metals plant in Mariupol from Ukrainian forces by first blocking all the exits out of the factory. “And then we’ll use chemical troops to smoke them out of there,” he said.

A Ukrainian regiment, without evidence, also claimed Monday that a drone had dropped a poisonous substance in Mariupol. It indicated there were no serious injuries.

Pentagon spokesman John Kirby said in a statement that the U.S. could not confirm the drone report out of Mariupol. But Kirby noted the administration’s persistent concerns “about Russia’s potential to use a variety of riot control agents, including tear gas mixed with chemical agents, in Ukraine.”

Russian forces will likely try to encircle the Donbas region from the north and the south as well as the east, said retired British Gen. Richard Barrons, co-chair of the U.K.-based strategic consulting firm Universal Defence & Security Solutions.

The ground in that part of Ukraine is flatter, more open and less wooded — so the Ukrainian ambush tactics used around Kiev may be less successful, Barrons said.

“As to the outcome, it’s finely balanced right now,” Barrons said. If the Russians learned from their previous failures, concentrated more force, connected their air force to ground forces better and improved their logistics, he said, “then they might start to overwhelm the Ukrainian positions eventually, although I still think it would be a battle of enormous attrition.”

Questions remain about the ability of depleted and demoralized Russian forces to conquer much ground, after determined Ukrainian defenders repelled their advance on Kyiv.

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Britain’s Defense Ministry said Monday that Ukraine has already beaten back several assaults by Russian forces in the eastern Donetsk and Luhansk regions — they make up the Donbas — resulting in the destruction of Russian tanks, vehicles and artillery.

Western military analysts say Russia’s assault increasingly is focusing on an arc of territory stretching from Kharkiv, Ukraine’s second-largest city, in the north, to Kherson in the south.

A residential area in Kharkiv was struck by incoming fire on Monday afternoon. Associated Press journalists saw firefighters putting out the fire and checking for victims following the attack, and saw that at least five people had been killed, including a child.

Oleh Synyehubov, the regional governor of Kharkiv, said earlier Monday that Russian shelling had killed 11 people over the last 24 hours.