Graves of civilians killed during the Ukraine-Russia conflict in the southern port city of Mariupol, Ukraine. Reuters

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.

The mayor of the Ukrainian port city of Mariupol said on Monday that more than 10,000 civilians have died in the Russian siege of his city, and that the death toll could surpass 20,000, as weeks of attacks and privation leave the bodies of Mariupol's people "carpeted through the streets.”

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Speaking by phone with the media, 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.

An armoured vehicle of pro-Russian troops is seen in the southern port city of Mariupol on Monday. Reuters

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.

Pro-Russian troops drive an armoured vehicle in Mariupol, Ukraine, on Monday. Reuters

Elsewhere, US 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.

Associated Press