First let them take Mariupol. It can take many months. And I am not sure whether the NAF is having enough resources to hold Mariupol. Remember that earlier they had withdrawn from Slavyansk-Kramatrosk-Krasnyi Liman for the lack of manpower. But I think if they take control of Mariupol, then they will hold it by any means possible. Mariupol is strategically very important.

Once they take Mariupol, I am not sure whether they will expand further. People's Republic of Zaporozhye was formed recently, but so far only a few hundred locals have shown any interest.