Russia's defenses are proving effective in frustrating the Ukrainian counteroffensive.
Its use of mines, adaptable drones, and electronic warfare are particularly formidable, experts said.
He said there were often more than four rows of minefields in front of the Russian defensive lines.
A retired Australian general Mick Ryan appears to agree with Lewis, telling The Economist on Monday that Russia's defensive lines are "much more complex and deadly than anything experienced by any military in nearly 80 years."
Justin Bronk, a senior research fellow with RUSI, told Newsweek that Ukraine's "Beaver" drones appear to be vulnerable to Russia's electronic defense systems.
Persons:
Oleksandr Tarnavskyi, Maxar David Lewis, Mick Ryan, Lewis, it's, Steve Wright, Wright, ALEXANDER NEMENOV, Justin Bronk, RUSI, Russia doesn't, Mark Milley, Birmingham's Jaroslava Barbieri
Organizations:
Service, BBC, Royal United Services Institute, Storm, Ukrainian, Police, Moscow International Business, Getty Images, Newsweek, Moscow Sunday, Reuters, Associated Press, University, Birmingham's, Ukraine's Ministry of Defense
Locations:
Russia, Wall, Silicon, Ukraine, Russian, Kherson, Dnipro, Russia's, Australian, RUSI, AFP, Ukrainian, Kyiv