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(AFP) - The screeching artillery shells crashing down from Russian positions in the embattled and rubble-strewn city of Bakhmut in east Ukraine shake 20-year-old sapper Denis the most.

"You hear a boom, a whistle and then an explosion. And of course the shrapnel," the military engineer told AFP, just hours after rotating from the fiercest battle of Russia's invasion.

"That's the worst. It's scary -- scary for anyone," he said, adding he had no antidote to fear other than staying home.

That's not an option for the military engineers gathered in a leafy courtyard several kilometres west of Bakhmut, who say the ferocious battle for the city is becoming precarious.

Their testimony to AFP offers a look inside the longest and bloodiest stand off of Russia's invasion, with access otherwise limited by heavy shelling and close street battles.

"The city looks more like ruins. There are almost no whole houses left. It's practically wiped out. There's shelling every day and every day there are more ruins," said Denis, who joined the military last year after university.

"Artillery and mortar shelling -- don't stop. Drones drop grenades. And of course there is street fighting and machine gun fire."

- 'No margin for error' -

Ukrainian authorities say fighting is on against slowly advancing Russian forces in the devastated city's central districts.

"In some places, the occupiers and the Ukrainian defenders are a few metres apart. There are no signs that Wagner is starved for shells," authorities said Tuesday, referring to the mercenary group leading Russia's charge.

The unit taking a short reprieve outside Bakhmut is tasked with demining Russian explosives and booby traps so Ukrainian infantry can move forward.

"When you go out, you don't know if you're coming back," said Oleg, 34, who works in positions around Bakhmut, often at night, often by moonlight.

"They even mine the corpses of their soldiers. When they retreat, they mine absolutely everything," he added.

"You have no margin for error. It all depends on how lucky you are."

Analysts say Bakhmut -- if there's anything left to salvage after the battle -- offers little strategic leverage. But both sides have heavily invested politically.

The Kremlin wants to sell a victory at home after a dry spell of several defeats.

- 'They don't follow rules' -

Kyiv says its military is holding out to exhaust Russian forces.

"More than once we have seen them jump out of cover and begin to run. They are killed, they gather their strength, and do it again," said Pavlo, a 33-year-old former firefighter.

Wagner's founder has hinted his forces might be running out of steam and twice in the last week called on Moscow to entrench rather than attack.

Pavlo conceded that despite Russian losses, the engineers he is competing with are formidable.

"I'll tell you this, they have very smart sappers. Say what you want -- very smart sappers.

"They don't follow any of the rules. Tripwires, certain anti-personnel mines -- they're banned but they plant them anyway."

Bakhmut, a town that has grown infamous over recent months of the war, was once known locally for its salt mines and sparkling wine production.

Around 70,000 people once called it home. Despite the bloodshed and bombing, some still do.

"Yesterday I saw an old woman. And I was really surprised. Everything was flying and whistling, and she was chopping firewood near the house. I guess they're already used to it," said Denis.

There are doubts about how much longer Ukrainian forces can hold out in Bakhmut but Denis believes that his brigade will be there for a long time.

"This is one of the most difficult areas right now. And there is a lot of work to do," he said.

© Agence France-Presse