ISW: Russia gains ground in Kostiantynivka but Fortress Belt stays out of reach
Russian forces have made fresh tactical advances into Kostiantynivka, the Institute for the Study of War (ISW) assessed on 10 June. The city sits at the southern tip of Ukraine's Donetsk Oblast Fortress Belt — Moscow's main effort for the spring-summer 2026 offensive. Russia missed its own May deadline to take the city, and the wider fortified chain stays out of operational reach.
Two Russian tactical groups push into eastern Kostiantynivka
Two named Russian formations have pushed into eastern Kostiantynivka from the south, Ukrainian military observer Kostiantyn Mashovets reported on 10 June. He identified them as the "Bakhmut" tactical group and the "Dzerzhinsk" (Toretsk) tactical group. The "Bakhmut" group is built around Russia's 3rd Army Corps (AC) under the Southern Grouping of Forces. The "Dzerzhinsk" group operates in the area of responsibility of the 8th Combined Arms Army (CAA) of the Southern Military District. It likely includes elements of five CAAs, the 3rd AC, and Russian naval fleets, Mashovets noted.
Elements of the "Bakhmut" group pushed from Stupochky through Novodmytrivka into northeastern Kostiantynivka. They also advanced along the T-0504 Pokrovsk-Bakhmut road as far as the city's railway station. The "Dzerzhinsk" group moved from Illinivka, south of the city, into areas stretching from northwestern to southwestern Kostiantynivka near the railway station. Mashovets assessed that it has likely achieved a tactical breakthrough in the western, central part of the city. Forward assault elements of the two groups now stand roughly two kilometers apart. Russian forces have so far failed to seize the railway station. Ukrainian troops cleared the village of Dovha Blaka southwest of the city of Russian infiltrators.
Eight months of grinding, one missed deadline
ISW noted that Russian forces opened the campaign for Kostiantynivka in August 2025 after seizing the majority of Chasiv Yar and Toretsk, with Toretsk alone running to roughly 26,000 Russian casualties. The first Russian troops infiltrated Kostiantynivka itself in October 2025. Russia has since worked into at least 12.69% of the city. Ukrainian officials reported earlier this spring that the Russian command had set a May 2026 deadline for the seizure. That deadline has come and gone.
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Russia has poured forces into the effort regardless. Ukrainian Commander-in-Chief Oleksandr Syrskyi said on 2 May that Russian activity in this direction noticeably increased in April. Russian units in the area had reportedly been replenished by 80% as of 6 June, ISW noted. The Russian command reportedly redeployed elements of the 70th Motorized Rifle Division to the Kostiantynivka-Chasiv Yar area last December. The redeployment likely came in preparation for the spring push.
Tactical gains likely, the Fortress Belt still out of reach
ISW assessed that Russian forces will likely keep infiltrating throughout Kostiantynivka. They will likely consolidate positions in parts of the city while suffering high casualties. Russia's 3rd AC northeast of the city appears to be struggling to dislodge Ukrainian forces from Chasiv Yar. That inhibits any move to envelop Kostiantynivka from the north. A Russian milblogger claimed on 9 June that Ukrainian forces recently counterattacked near Chasiv Yar. The milblogger added that Ukraine still holds Podilske and Mykolaivka west of the town.
The northern flank of the Fortress Belt is also bogged down. Russia opened its spring-summer 2026 offensive with mechanized assaults around Lyman. Those assaults signaled intent to advance on Sloviansk from the northeast. They produced no significant gains, ISW noted. The Russian command likely shifted weight south to Kostiantynivka. Russia's Western Grouping of Forces covers the front from Kupiansk through Lyman. It likely lacks the combat power to push on Sloviansk while balancing operations toward Kupiansk and Borova.
