Bakhmut (Ukraine) (AFP) - With Russian artillery exploding around him, a Ukrainian soldier takes shelter in a tunnel and recounts the scene at the front line in Bakhmut, just a kilometre away.
"It's horror over there.The ground is black like asphalt.Everything is destroyed," 38-year-old serviceman Yevgen told AFP.
"There are bodies everywhere".
The eastern Ukraine town, known for its salt mines and vineyards, has been under attack for months by Russian forces, who are mostly on the defensive in other regions across Ukraine.
A driving force behind the offensive is the Wagner mercenary group, say war observers and Ukraine troops.
The shadowy force was founded by Kremlin-linked businessman Yevgeny Prigozhin.
The 61-year-old has taken an increasingly prominent role in Russia's invasion of Ukraine -- leading some to question if he might harbour personal political ambitions.
Ukrainian officials say Prigozhin has been sending thousands of soldiers recruited in Russian prisons to the front line, with the promise of a salary and an amnesty.
Several Ukrainian soldiers in Bakhmut told AFP these alleged ex-convicts are used as a type of "human bait".
"It starts at around 6:00 pm, when it's getting dark," said Anton, a 50-year-old Ukrainian soldier from the 93th brigade who was resting after an injury.
"These soldiers -- with no experience -- are sent towards our guns and stay there for a few minutes," Anton, nickname "Polyak", told AFP.
He estimated seven or eight were sent every night.
"Their job is to advance towards us, forcing us to fire on them, to reveal our positions," said Sergiy, a major in the 53rd brigade.
"After that, they fire artillery or send more experienced commandos towards our positions."
'Number of your prison'
Most of these fighters -- dubbed "disposable soldiers" by the Ukrainians -- are killed, they said.Some are wounded and captured.
Sergiy said he found one alive one morning.
In a video he said he took and showed to AFP later on the same day, a Russian captive is seen lying on the ground in a room, his right arm and left leg injured, being interrogated by the major.
"What is the number of your prison?" the major is heard asking in the video.
"Kopeika", the man says, giving the nickname of a prison in Voronezh in western Russia.
The Russian mercenary then says he joined Wagner one month earlier and received training in three different places, including a Russian-occupied area in eastern Ukraine.
The man is heard saying that all his fellow fighters were ex-inmates recruited by Wagner.
AFP could not independently confirm the contents of the video.
'Getting you out of here'
The mercenary group emerged in 2014 in Ukraine and is suspected by the West of doing the Kremlin's dirty work in countries like Syria and the Central African Republic -- a charge that Russia has always denied.
Apart from prison recruits, the private company also employs hardened mercenaries and professional soldiers, who work for Wagner because they are better equipped and better paid than in the regular Russian army.
Prigozhin only last month put an end to years of rumours and acknowledged he founded Wagner.
A few days earlier, a video had emerged in which a figure strongly resembling Prigozhin was seen recruiting prisoners from a Russian jail to send them to fight in Ukraine.
"I'm getting you out of here alive.But I won't always get you back alive," the figure is heard telling a crowd of detainees.
Prigozhin's company Concord did not confirm or deny whether he was the man in the video.
'Pyrrhic victory'
Experts say recruiting prisoners is a sign of weakness by Moscow, following battlefield setbacks and the huge controversy surrounding the drafting of some 300,000 Russian citizens.
According to Ukrainian President Volodymyr Zelensky, Prigozhin may have recruited up to 2,000 former prisoners to fight.
The other question for experts is why the mercenary boss would fight so hard for a city with little strategic value.
"The Russians are exhausting themselves without managing to surround or destroy enemy positions," said Mykola Bielieskov from Ukraine's National Institute for Strategic Studies.
Retired Ukrainian colonel Sergei Grabsky said that "technically, Russia can capture Bakhmut -- but not in the near future" and it would be a "Pyrrhic victory" because of the heavy losses.
The experts point to a possible political value for Prigozhin in presenting himself as more effective than Russia's generals.
"Russian troops are on the defensive and he is showing himself as being on the offensive.
"That is his main interest -- to transform this battle into political influence and therefore money," Bielieskov said.
For Nestor, a soldier from the 53rd brigade, Prigozhin is living up to his nickname as "Putin's chef" because his company provides the catering for Kremlin kitchens.
"He is turning 1,000, 2,000, 3,000 soldiers into cannon fodder," the soldier said.
