Russia likely lacks the reserves to properly defend itself if Ukraine breaks through a key defensive line, experts say

• Russia's forces appear to be stretched along its multi-layer defenses, a US think tank said. 
• The same units appeared to be deployed in both advanced and defensive roles, said the ISW. 
• Russian forces are coming under significant pressure from Ukrainian attacks in Zaporizhzhia. 

Russian defenses are probably lacking strength in depth on a key part of the front line where Ukraine has recently made important gains, data from a US think tank indicates.