One City Stands Between Russia and the Rest of the Donbas. Its name is Kostiantynivka, and the build-up around it is enormous.

On June 22, 2026, the BBC reported that Russian forces had infiltrated Kostiantynivka, a battered industrial city in Ukraine’s Donetsk region, and were maneuvering to surround it. Ukrainian soldiers told the broadcaster that the entire city has become a “grey zone” — no longer firmly held by either side.

The reason this one mid-sized town matters out of all proportion to its size is geography. Kostiantynivka is the southern anchor of Ukraine’s “fortress belt,” the chain of fortified cities that has held the line in the Donbas for years. If it falls, the road opens toward Kramatorsk and Sloviansk — Ukraine’s last major strongholds in the east — and Moscow moves a long step closer to one of its oldest war aims: seizing all of the Donbas.

But the headline build-up has not, so far, translated into a fast advance. The Institute for the Study of War and Ukrainian commanders describe a slow, costly grind — in some places measured in tens of meters a day. This page lays out what is actually happening around Kostiantynivka, what the numbers say, and what is still genuinely uncertain.

• Oct. 25, 2025 — the day Russia's push on Kostiantynivka began, following its August 2025 capture of nearby Toretsk · Source: Battle of Kostiantynivka chronology
• ~130–250 troops — Russian soldiers reported operating inside Kostiantynivka by mid-June 2026, up from small infiltration groups · Source: BBC; ISW
• 4 fortress cities — Kostiantynivka, Druzhkivka, Kramatorsk and Sloviansk — the Donbas 'fortress belt,' prewar population over 380,000 · Source: Institute for the Study of War
• 50% vs ~5% — Russian chief of staff Valery Gerasimov's claim of control over Kostiantynivka, against ISW's independent estimate · Source: ISW; Battle of Kostiantynivka
• ~8,000 civilians — remaining in a city of roughly 65,000 before the full-scale invasion — most have fled the fighting · Source: Euromaidan Press; ISW

Kostiantynivka sits in central Donetsk Oblast, a railway and industrial town that before the 2022 full-scale invasion was home to roughly 65,000 people. Today the BBC and Ukrainian aid groups put the number of remaining civilians at around 8,000. Its value is not its factories — it is its position. Kostiantynivka is the gateway through which a southern Russian advance would have to pass to reach the twin cities of Kramatorsk and Sloviansk, the administrative and logistical heart of the part of Donetsk that Ukraine still controls.

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Russia’s drive on the city is the natural continuation of a years-long crawl westward. After capturing Bakhmut in 2023, Avdiivka in 2024, and Toretsk in August 2025, Russian forces turned toward Kostiantynivka, with the first infiltration attempts into its southeastern edge logged on October 25, 2025. Taking it would, in the Kremlin’s framing, finally make good on the constitutional annexation it declared over the entire Donetsk region in 2022 — territory it has never fully held.

What changed in the spring and summer of 2026 was scale and intent. ISW assesses that capturing Kostiantynivka became the main objective of Russia’s 2026 spring–summer offensive. Russian forces escalated from small probing groups in late 2025 to roughly 130 to 250 soldiers operating inside the city by mid-June 2026, according to the BBC and ISW — a shift from infiltration to sustained urban combat. The units credited with the push include elements of Russia’s 70th Motorized Rifle Division and the 4th and 72nd Motorized Rifle Brigades.

The approach has been a pincer. Russian assault groups pushed from the southwest along the H-20 Donetsk–Kramatorsk highway and through the suburb of Illinivka, while a second axis ground up the city’s large Zinc Plant industrial zone along the Kryvyi Torets River that bisects the town. The Russian Ministry of Defense has claimed its troops are advancing in the southwest and have encircled elements of Ukrainian brigades — claims Ukraine denies. ISW, for its part, repeatedly logged Russian “infiltrations” near the city without confirmed advances on several of the key dates.

To understand why Kostiantynivka is worth this much blood, look at the line behind it. The “fortress belt” is a chain of four large cities — Kostiantynivka, Druzhkivka, Kramatorsk and Sloviansk — running roughly 50 kilometers (about 31 miles) north to south along the H-20 highway, with a prewar population that ISW puts at over 380,000 people. For years Ukraine has poured time and money into fortifying this corridor with bunkers, trenches, tank traps, minefields and concrete “dragon’s teeth,” using terrain with steep slopes that favors the defender.

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Kostiantynivka is the belt’s southern gate. Russian planners have made no secret that breaking the belt is the prize: control Kostiantynivka, and an advance could swing north toward Druzhkivka and Kramatorsk or attempt to outflank the line and cut the supply roads that keep the whole defense alive. That is why analysts describe the city as a “backbone” of Ukraine’s eastern defense rather than just another town on the map — and why its loss would be a strategic event, not a local one.

For all the talk of a build-up, the advance itself has been agonizingly slow. A Ukrainian officer told the BBC the Russians sometimes move only 100 meters a day — “sometimes they even crawl to reach the next building.” The reason is drones. Both sides now blanket the battlefield with first-person-view drones, turning the open ground around the city into a “kill zone” that small Russian groups can only cross by hiding in buildings and tree lines. ISW noted in June that Ukrainian drone activity in the neighboring Pokrovsk direction was actively hindering Russian troop rotations and limiting their ability to advance.

The gap between claim and reality has been just as stark. Russia’s chief of the General Staff, General Valery Gerasimov, claimed Russian forces controlled roughly half of Kostiantynivka; ISW’s independent mapping put confirmed Russian control at closer to 5 percent. Russian forces also missed an earlier internal deadline to take the city and, as of the June reporting, had not seized its central rail hub. ISW assessed that while Russia may establish footholds inside Kostiantynivka over the summer, doing so would come at enormous cost, and breaking the entire fortress belt this year remained unlikely.

“Sometimes they move 100m a day. Sometimes they even crawl to reach the next building.”

Ukrainian officer near Kostiantynivka, to the BBC (June 2026)

Ukraine has rotated some of its better mechanized formations through the city’s defense, including its 24th, 28th and 93rd Mechanized Brigades and the 129th Heavy Mechanized Brigade. The officer commanding the sector, Brigadier General Oleksandr Bakulin, who leads Ukraine’s 19th Corps, told the BBC the situation “remains under control” and that “the enemy has no success” — while acknowledging the Russian presence inside the city. Ukrainian forces have kept up “clean-up and assault” operations against the infiltration groups even as more Russian soldiers accumulate.

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Ukrainian commanders also argue the wider picture is less grim than the city-by-city headlines suggest. Kyiv says it has retaken more ground than it lost across the front this year, and its long-range strikes on Russian oil refineries have created fuel shortages that ripple back to the front — including reported halts in fuel sales in occupied Crimea. Meduza’s June analysis concurred that, contrary to Putin’s claim of advancing on every front, the decisive fight is concentrated around Kramatorsk, and Ukraine could plausibly hold the city through 2026.

A front-line story this fast-moving deserves honest caveats. Troop counts inside a contested city are estimates, not census figures; the “130 to 250” range reflects that uncertainty. Both governments have an incentive to spin — Russia to project momentum, Ukraine to project control — so the safest readings come from independent mappers like ISW and from reporters physically near the line, such as the BBC and Kyiv Independent teams. Whether Kostiantynivka actually falls “by the end of summer,” as one Ukrainian observer warned, is a forecast, not a fact.

What is solid is the shape of the thing. Russia has concentrated its main 2026 effort on Kostiantynivka. Russian troops are inside the city and the surrounding zone is contested. The city is the southern gate of a fortified belt whose loss would expose Kramatorsk and Sloviansk. And the advance, for now, is slow and expensive — with Gerasimov’s “half the city” claim contradicted by independent assessment. Those facts are not in serious dispute; the timeline is what remains open.

Kostiantynivka is where the war’s biggest stakes have narrowed to a single town. Hold it, and Ukraine’s fortress belt stays intact and the Donbas stays contested. Lose it, and the road to Kramatorsk and Sloviansk — and to Moscow’s long-stated goal of taking all of Donetsk — grows shorter. For now the build-up is real but the breakthrough is not: Russian soldiers crawl building to building under a sky full of drones while their own commander overstates how much they hold. We’ll keep tracking the ISW maps, the line of contact, and whether the slow grind around Kostiantynivka turns into something faster.

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Last updated June 23, 2026