Russian advances could give it a shot at Ukraine's eastern 'fortress belt,' war analysts warn

• Russian forces are poised to advance toward the "fortress belt" of four eastern Ukrainian cities.
• The cities are a stronghold for Ukraine in Donetsk Oblast just northwest of Avdiivka.
• Russia has been making gains in the area, forcing Ukrainian troops to withdraw. 

Russian troops appear to be making tactical advances just northwest of Avdiivka toward a so-called "fortress belt" of Ukrainian cities, war analysts report.