What flag is that?
Donetsk People’s Republic , supposedly. One of the 2014 breakaway states the Russians carved out of Ukraine.
Wikipedia article: Donetsk People's Republic.
You may remember that a few days before the Russians invaded Ukraine that President Putin recognized the two break-away states (the other being the Luhansk People's Republic) as a prelude to the invasion.
Here's the flag of the Donetsk People's Republic, per Wikipedia:
Map of the territory controlled and claimed by the DPR:
I believe Mariupol is the light green dot surrounded by dark green, on the Black Sea, towards the bottom of the map.
Apparently it is associated with the Donetsk People's Republic...
;>)
Just speculation, it might be the Russian Flag used for this particular Military Campaign to distinguish it from the standard Russian Federation tricolor.
Pro-Russia separatist militia.