Russian troop build-up threatens city seen as key to seizing Ukraine's Donbas

Russian troops have infiltrated the strategic city of Kostyantynivka in eastern Ukraine and are now trying to surround it.

The entire city is now effectively in a "grey zone", no longer controlled by anyone, Ukrainian soldiers have told the BBC.

"They get into areas behind our backs and in urban conditions it's extremely difficult to push them out," says a Ukrainian drone pilot who operates in that area and prefers to remain anonymous.

Kostyantynivka is a gateway to the rest of the Donbas region.

If it falls, Russian forces would be able push towards Ukraine's last remaining strongholds in the east, the cities of Kramatorsk and Sloviansk, and move closer to seizing Donbas completely, one of the Kremlin's key objectives in this war.

For months Russia's full-scale war in Ukraine has stalled along the front line, and Ukrainian commanders say they have recaptured more territory this year than they have lost, disrupting Moscow's crucial supply lines between the Russian border and occupied Crimea. On Sunday, Russian-appointed authorities in Crimea suspended fuel sales to the public because of the shortages.

And Kyiv's repeated strikes on oil refineries in Moscow, St Petersburg and other cities have also sought to bring the conflict to the attention of Russians far from the front line.

But in Kostyantynivka, Russian soldiers have advanced from the south and have even been spotted at the other end of the city, on the city's northern outskirts.

Moscow says its forces are rapidly advancing in the south-western part of Kostyantynivka and that they have encircled Ukrainian military units.

That is denied by Kyiv and Brig Gen Oleksandr Bakulin, commander of Ukraine's 19th Corp which is in charge of units defending the city and surrounding areas, insists "the situation remains under control" and "the enemy has no success".

However, he does acknowledge there are about 130 Russian soldiers still inside the city.

The situation on the ground may not be as critical as Moscow says, but it does appear far more serious than Ukrainian officials are ready to publicly admit, a Ukrainian officer from a unit that operates inside Kostyantynivka told the BBC.

He too has preferred to stay anonymous, but told the BBC: "We still have clean-up and assault groups inside the city, but the Russians manage to accumulate more and more soldiers there."

Every building in Kostyantynivka acts as a potential shelter for soldiers to hide, and in the summer months the trees provide extra cover.

So Russian forces have found a way to advance inside the "kill zone" where Ukrainian drones can detect any movement and instantly attack.

Russian drone pilots have also focused on destroying Ukrainian drone launch sites.

To give their infantry troops assaulting Ukrainian positions a breathing space, Russian pilots had made Ukrainian crews their priority target.

The Ukrainian drone pilot told the BBC that while his colleagues were targeting Russia's infantry, enemy crews were not: "For such tasks, they use artillery, multiple rocket launchers, military aviation."

He and his comrades have limited numbers and resources, so they are often exhausted and have struggled to expand their operations.

"Since we devote little time to [searching and targeting] enemy pilots, they can freely operate, detect our positions and we are forced to roll back. This is how the frontline moves," he explains.

The Ukrainian officer whose unit is operating in the city maintains the pace of the Russian advance is very slow: "Sometimes they move 100m [109yds] a day. Sometimes they even crawl to reach the next building."

Russian forces in Kostyantynivka have adopted a similar strategy to that used in the capture of Pokrovsk and other major cities in eastern Ukraine, advancing along the flanks to surround the city and cut off supply routes.

In recent days the Russian defence ministry has said it has seized villages to the west of the city.

Such reports have helped divert attention from Ukraine's attacks on oil refineries deep inside Russia and on supply routes to Crimea, and the major fuel shortages they have caused.

That appears to explain the Kremlin's decision to surround and seize Kostyantynivka as quickly as possible.

The situation has become a "major crisis", the Ukrainian drone pilot warns.

Russian drone units do not even need to rely on sophisticated unmanned systems as they are now close enough to use cheap Chinese drones that cannot fly far to conduct surveillance and identify Ukrainian drone launch sites.

Delivering supplies has become extremely difficult, with land routes constantly targeted and big logistical drones regularly intercepted.

The fall of Kostyantynivka "is a matter of time", says Ukrainian frontline monitoring project DeepState.

Once that happens, logistical operations in the area will be more complicated and even remaining in Kramatorsk will become "extremely dangerous", it reports.

Another Ukrainian soldier whose unit is fighting in the city has said they are not getting the necessary reinforcements to cover their losses, and their commanders are reluctant to report about lost positions.

If they do, then they will be ordered to retake them: "We don't have enough people to hold on to what we still control, let alone organise assaults."

"We are still fighting inside the city," the drone pilot says, "But unless we change our approach and start destroying their logistics and targeting their pilots, they will continue advancing."