It is not clear if Russia is using mobile crematoriums. There is an image circulating on social media claiming this, but it is actually from 2013.
A Twitter post by NEXTA TV quoted the mayor of Mariupol, Vadym Boichenko, claiming Russian mobile crematoria have started to operate in the city to cover up war crimes. His words implied that the method was used recently to hide casualties by the enemy. The post also included an image of a crematorium mounted on a vehicle to further the claim. Mariupol City Council officials have accused Russia that thousands of civilians have been killed as the city has been bombarded relentlessly since the start of the invasion on February 24.
A reverse image search shows the footage in question dates back to 2013, posted on YouTube by a Russian incinerator construction company Turmalin ZAO. The blurb on the video says, "Testing of a mobile crematorium for biological waste." Nearly every media reporting about this quotes Mariupol city council officials and the city mayor as their sources, but none of them has been independently able to verify the presence of such mobile crematoriums in Mariupol during the current war.
Business Insider cited the Bloomberg report of 2015 that Russian forces have previously used such mobile crematoriums to burn the bodies of dead Russian soldiers who were fighting alongside pro-Russian separatists in Eastern Ukraine. News claiming the use of such mobile crematoriums in Mariupol during the current war in Ukraine to burn the victims' bodies has been reported. However, we could not independently verify these allegations.
We have marked this claim as unverifiable as there is a lack of evidence on the usage of mobile crematoriums by Russian forces in Mariupol. More information on the matter can be expected in the days to come.