Fall is expected to bring another gruelling test for Ukraine’s armed forces as Russia intensifies its campaign to seize an eastern region it has sought to conquer for years.

Russia now controls about 70% of the Donetsk region, once Ukraine's industrial heartland.

Ukraine's forces have been pushed back to a string of four cities that analysts have dubbed its “fortress belt,” where they've pushed back Moscow's efforts to seize the region for years.

But shortages of troops, supplies and chaos in management are making it increasingly hard to resist Russia's relentless pressure in the region.

As the invasion continues despite months of US-led peace efforts, analysts say the country could struggle to resist an intensified push to seize the last cities in the region under Ukrainian control.

Analysts and Ukrainian officers also say Russia is unlikely to engage in protracted urban battles and avoid costly fights like the storming of Bakhmut, which dragged on for months and inflicted staggering losses on both sides.

To skirt the cities, Russian forces are pushing on the flanks and increasingly using infiltration tactics, sending small groups of soldiers through gaps between Ukrainian units.

Some of these groups have achieved tactical gains, slipping behind the front line to hide in tree lines or basements, occasionally occupying abandoned positions or cutting off supply routes.

But the human cost is heavy. Of a five-man unit, Ukrainian commanders estimate, two are usually killed, one is wounded, one goes missing, and only one survives to call for a drone to drop water or medicine.

Drones and glide bombs are also playing a crucial role, allowing Russia to hit troops and supplies headed for the front and to weaken Ukraine's strongholds without head-on fighting.

This summer, Russian forces stepped up attacks at the northern and southern ends of the Donetsk fortress belt.

Their strategy, Ukrainian officers say, is to sever supply lines and surround the region’s cities rather than storming them directly.

The region was one of four that Russia illegally annexed in 2022, though it didn't control any of them.

However, the Donetsk region became the epicentre of the frontline throughout Moscow's invasion.

In the north, Yurchuk's 63rd Brigade is fighting to hold the city of Lyman, a key logistics hub on the way to Sloviansk.

Lyman has rail connections, dozens of basements and bomb shelters, solid infrastructure and strong buildings where command posts or supply depots could be set up.

The city was occupied during part of the first year of the full-scale invasion but liberated in Ukraine’s autumn 2022 lightning counteroffensive.

If Russian forces manage to take the city, commander of Ukraine’s 63rd Brigade Pavlo Yurchuk said, they could use it to build up troops and attempt to cross the Siverskyi Donets River, a natural obstacle that helps protect Sloviansk.

But the commander says he's confident Russia's latest offensive won't work.

“From a military point of view it looks correct — on the map it looks neat — but after nearly three and a half years of war we all know that such deep maneuvers and wide flanking operations are not Russia’s forte,” said Yurchuk. “They simply won’t be able to control and supply those penetrations, so I’m sure that they will fail.”

In southern Donetsk, Russia has made advances near Pokrovsk, taking them further around the fortress belt's southern stronghold of Kostiantynivka, once home to 67,000 people but today all but deserted.

It’s hard to predict how the fighting will unfold: Russia could suddenly break through and seize territory, or the battle for the region could drag on for months or years.

While Russia achieves tactical gains without sparing sentiment for human life, Ukraine faces the grinding reality of troop shortages.

AP video by Vasilisa Stepanenko