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.
The Institute for the Study of War said Tuesday that units from the same Russian divisions were deployed in Tokmak, a village just behind the front line that's a key target of Ukraine's counteroffensive, as well as in more advanced positions fighting Ukrainian forces.
The ISW said this suggested Russia had not manned its multi-echeloned defense in southern Ukraine in depth.
"The deployment of the 70th and 71st Motorized Rifle Regiments as far back as Tokmak suggests that elements of the same Russian formations and units defending at forward positions are holding positions, likely in smaller numbers, in subsequent defensive layers," it said.
It added it was possible that elements of other Russian units and formations held rear defensive positions, but the current Russian manpower commitment to holding positions on the frontline "indicates that this is unlikely."
Russian forces are coming under intense pressure from Ukrainian attacks in the southern Ukrainian Zaporizhzhia Oblast, with the ISW in other recent updates saying that Russian forces were stretched thin and elite units were sustaining high casualties during counterattacks.
Ukraine recently took control of the town of Robotyne, beyond Russia's initial defensive lines, and is targeting Tokmak, which lies on a strategically important road.
The ISW's assessment echoes that of other analysts, who have said Russian defenses beyond the initial, heavily fortified front two lines appear to be weak, suggesting that Ukraine could make rapid advances if it manages to break through.
Ukraine is probing Russian defenses at several points simultaneously along the heavily fortified "Surovikin line" in a bid to expose weak spots and force it to deploy elite units thinly, reports have said.
One of the core aims of Ukraine's counteroffensive in south Ukraine is to retake the strategic town of Melitopol, which would enable it to significantly disrupt supplies traveling to the Russian-occupied Crimea Peninsula by road and rail.
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