More than 10,000 civilians have been killed in the besieged city of Mariupol since the Russian invasion of Ukraine in February, the mayor said, as Western nations warned a convoy was on the move for a suspected Russian assault in Ukraine’s east.

The city is crucial to Russia's effort to link Crimea with the Donbas region, where Moscow-backed separatists have established de facto republics that even Russia only recognized days before the war broke out in February.

Mariupol Mayor Vadym Boychenko told The Associated Press that Russian forces have blocked weeks of attempted humanitarian convoys into the city in part to conceal the carnage. Boychenko also said the death toll in Mariupol alone could surpass 20,000

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

The Pentagon on Monday evening acknowledged social media reports of Russian forces detonating a chemical weapon in Mariupol but could not confirm whether they're accurate, Pentagon press secretary John Kirby said in a statement.

"These reports, if true, are deeply concerning and reflective of concerns that we have had about Russia’s potential to use a variety of riot control agents, including tear gas mixed with chemical agents, in Ukraine,” Kirby said.