[Vlad0]

That is the flag of one of the two break-away Republics in Eastern Ukraine, the Donetsk Peoples Republic:

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.

[EEGator]

Gotcha. So are they stating their independence, their allegiance to Russia, or some type of hybrid?

[Lisbon1940]

Not a land bridge.