“They will come at Mariupol from two or more directions”
There have consistent reports of unrest in and around Mariupol, since this began. The Right Sector units are reported to have killed several civilians/seperatists in the area since the civil war started, as well.
just random speculation, but they could be waiting for some kind of an internal revolt, before trying to take the city.
Given that the defense is likely Right Sector, and some recently conscripted support troops, it could get really messy.