Russia is continuing to make slow but costly gains in the most politically significant terrain of eastern Ukraine, the Donetsk “fortress belt”, and now controls around 80% of the urban centre of Kostiantynivka, according to a Western official.

Speaking on the war in Ukraine, the official said Ukraine had achieved a number of tactical successes over recent weeks and months, but that in the fortress belt the initiative remained with Russia. “In the most politically impactful terrain, the fortress belt in Donetsk Oblast, that is where the Russians are still making gains, albeit, as we know, still torturously, slowly, and with excruciating casualties,” the official said.

The official described the fortress belt as a cluster of four towns, naming Sloviansk, Kramatorsk and Kostiantynivka among them. In the latter, “Russia now controls approximately 80% of the urban centre,” the official said, while cautioning that firm figures were difficult. “The very fragmented and intermingled character of the front line means that any metric of territorial control is at best imprecise, and, as we know, it remains drone dominated.”

Russia’s defence ministry declared earlier in July that Kostiantynivka had fallen, a claim contradicted by Ukrainian troops still fighting inside it. The independent Russian outlet Meduza, analysing the battle in early July, judged that both Moscow’s claim of capture and Kyiv’s insistence that it held the city were wrong, reporting that Russian assault troops had appeared in every part of Kostiantynivka except the western outskirts while Ukrainian forces continued to hold positions, including in the centre. The Institute for the Study of War has separately assessed that Ukraine’s ability to defend the city is deteriorating, while noting that Moscow’s detailed announcements were designed to create a greater impression of Russian presence than actually existed.

At the northern anchor of the belt, Sloviansk, the official said Russia continued to make incremental gains and was contesting the key pivot town of Lyman. That matches recent battlefield reporting: Meduza reported that Russian forces have pressed a determined assault on Lyman since late June, with assault troops appearing across much of the city while Ukrainian defenders hold on, and that separate Russian forces were pushing along the Siverskyi Donets river towards Sloviansk itself.

To the south-west, Russia was said to be heavily set on seizing Dobropillia, which the official described as “an important fulcrum town, which could potentially give Russia the platform and the ability to flank the fortress belt from the west.” Ukrainian military spokesman Victor Tregubov of the Khortytsia group of forces told the broadcaster Suspilne on 27 July that Russian troops had achieved a partial wedge between Pokrovsk and Kostiantynivka and were trying to develop it, with the Pokrovsk direction remaining the main vector of the Russian offensive.

The fortress belt has been central to Russia’s stated war aims since the full-scale invasion began in February 2022. The chain of urban areas running through Donetsk Oblast forms the most heavily fortified Ukrainian defensive line in the east, developed over more than a decade since fighting first broke out in the Donbas in 2014. Kramatorsk, the largest city in the belt, has served as the main administrative and military hub for Ukrainian-held Donetsk Oblast, and capturing the belt would give Moscow effective control of the remainder of the oblast, one of four regions Russia claims to have annexed in September 2022.

Kostiantynivka itself had a pre-war population of roughly 67,000 and has long been one of the key logistical hubs for the Ukrainian military in the east. Lyman, the pivot town the official identified as contested, changed hands twice earlier in the war, falling to Russia in May 2022 before being retaken by Ukraine that October during the Kharkiv counteroffensive.

Elsewhere along the front, the official reported little change. Russia’s attempt to create a buffer zone on the border of Sumy and Kharkiv oblasts was “only making very incremental inroads,” the official said, while in the south, below Zaporizhzhia city, Russia had “effectively deprioritised that sector,” although fighting remained heavy around Huliaipole.