LVIV, Ukraine — Creeping forward along a tree line late at night toward an entrenched Ukrainian position, the Russian soldier watched in horror as his comrades were mowed down by enemy fire.

His squad of 10 ex-convicts advanced only a few dozen yards before being decimated. “We were hit by machine gun fire,” said the soldier, a private named Sergei.

One soldier was wounded and screamed, “Help me! Help me, please!” the private said, though no help arrived. Eight soldiers were killed, one escaped back to Russian lines and Sergei was captured by Ukrainians.

The soldiers were sitting ducks, sent forth by Russian commanders to act essentially as human cannon fodder in an assault. There are two main uses of the conscripts in this tactic: as “storm troops” who move in waves, followed by more experienced Russian fighters, and as intentional targets, to draw fire and thus identify Ukrainian positions to hit with artillery.

Either way, they have become an integral component of Russia’s military strategy as it presses a new offensive in Ukraine’s east: relying on overwhelming manpower, much of it comprising inexperienced, poorly trained conscripts, regardless of the high rate of casualties.

In interviews last week, half a dozen prisoners of war provided rare firsthand accounts of what it is like to be part of a sacrificial Russian assault.

“These orders were common, so our losses were gigantic,” Sergei said. “The next group would follow after a pause of 15 or 20 minutes, then another, then another.”

Of his combat experience, he said, “It was the first and last wave for me.”

By luck, the bullets missed him, he said. He lay in the dark until he was captured by Ukrainians who slipped into the buffer area between the two trench lines.

The New York Times interviewed the Russians at a detention center near Lviv in Ukraine’s west, where many captured enemy soldiers are sent. From there, some are returned to Russia in prisoner exchanges. The Times also viewed videos of interrogations by the Ukrainian authorities. The prisoners are identified only by first name and rank for security reasons, because of the possibility of retribution once they are returned.

Though they are prisoners of war overseen by Ukrainians, the Russians said they spoke freely. Their accounts could not be independently corroborated but conformed with assessments of the fighting around the eastern Ukrainian city of Bakhmut by Western governments and military analysts.

The soldiers in Sergei’s squad were recruited from penal colonies by the private military company known as Wagner, whose forces have mostly been deployed in the Bakhmut area. There, they have enabled Russian lines to move forward slowly, cutting key resupply roads for the Ukrainian Army.

Russia’s deployment of former convicts is a dark chapter in a vicious war. Russia Behind Bars, a prison rights group, has estimated that as many as 50,000 Russian prisoners have been recruited since last summer, with most sent to the battle for Bakhmut.

In the early phases of  the war, the Russian Army had copious armored vehicles, artillery and other heavy weaponry but relatively few soldiers on the battlefield. Now, the tables have turned: Russia has deployed about 320,000 soldiers in Ukraine, according to Ukraine’s military intelligence agency. An additional 150,000 are in training camps, officials said, meaning there is the potential for half a million soldiers to join the offensive.

But using infantry to storm trenches, redolent of World War I, brings high casualties. So far, the tactic has been used primarily by Wagner in the push for Bakhmut. Last week, the head of Wagner, Yevgeny V. Prigozhin, said he would end the practice of recruiting convicts. But Russia’s regular army this month began recruiting convicts in exchange for pardons, shifting the practice on the Russian side in the war from the Wagner private army to the military.

Some military analysts and Western governments have questioned Russia’s strategy,  citing rates of wounded and killed at around 70 percent in battalions featuring former convicts. On Sunday, the British defense intelligence agency said that over the past two weeks, Russia had probably suffered its highest rate of casualties since the first week of the invasion.

Interviews with former Wagner soldiers at the Ukrainian detention center aligned with these descriptions of the fighting — and shed light on a violent, harrowing experience for Russian soldiers.

“Nobody could ever believe such a thing could exist,” Sergei said of Wagner tactics.

Sergei, sat, shoulders slumped, on the sofa in the warden’s office of the Ukrainian detention center. He was balding and wore shoes without laces.

The soldiers arrived at the front straight from Russia’s penal colony system, which is rife with abuse and where obedience to harsh codes of conduct in a violent setting is enforced by prison gangs and guards alike. The same sense of beaten subjugation persists at the front, Sergei said, enabling commanders to send soldiers forward on hopeless, human wave attacks.

“We are prisoners, even if former prisoners,” he said. “We are nobody and have no rights.”

Sergei said he had worked as a cellphone tower technician in a far-northern Siberian city, living with his wife and three children. In the interview, he admitted to dealing marijuana and meth, for which he was sentenced to 10 years in prison in 2020.

In October, he accepted an offer to fight in exchange for a pardon. The arrangement, he said, was not offered to rapists and drug addicts, but murderers, burglars and other prisoners were welcome.

“Of course, any normal person fears death,” he said. “But a pardon for eight years is valuable.”

The fighting would turn out to be far more dangerous than he had imagined.

In three days at the front south of Bakhmut, Private Sergei first served as a stretcher bearer, carrying out mangled, bloody former prisoners who had been killed or wounded in an omen of what awaited him when ordered to join an assault.

 On the night of Jan. 1, they were commanded to advance 500 yards along the tree line, then dig in and wait for a subsequent wave to arrive. One soldier carried a light machine gun. The others were armed with only assault rifles and hand grenades.

The sequential assaults on Ukrainian lines by small units of former Russian prisoners have become a signature Russian tactic in the effort to capture Bakhmut.

“We see them crawl for a kilometer or more,” toward Ukrainian trenches, then open fire at close range and try to capture positions, Col. Roman Kostenko, the chairman of the defense and intelligence committee in Ukraine’s Parliament, said in an interview. “It’s effective. Yes, they have heavy losses. But with these heavy losses, they sometimes advance.”

It could be, Colonel Kostenko said, that such infantry assaults on entrenched defenses will remain mostly confined to the fight for Bakhmut and that they are being used to conserve tanks and armored personnel carriers for the expected offensive. But they could also serve as a template for wider fighting.

The former convicts, Colonel Kostenko said, are herded into the battlefield by harsh discipline: “They have orders, and they cannot disobey orders, especially in Wagner.”

A private named Aleksandr, 44, who shaved three years off a sentence for illegal logging by enlisting with Wagner, said that before deploying to the front he was told he would be shot if he disobeyed orders to advance.

“They brought us to a basement, divided us into five-person groups and, though we hadn’t been trained, told us to run ahead, as far as we could go,” he said of his commanders.

His dash toward Ukrainian lines in a group of five soldiers ended with three dead and two captured.

Another captured Russian, Eduard, 22, enlisted to get four years cut from a sentence for car theft. He spent three months at the front as a stretcher bearer before being ordered forward. He was captured on his first human wave assault. From his time as a stretcher bearer, he said, he estimated that half of the men in each assault were wounded or killed, with shrapnel and bullet wounds the most common injuries.

Private Sergei said he had initially been pleased with the offer of a pardon in exchange for service in Wagner. “When I came to this war, I thought it was worth it,” he said.

But after his one experience in an assault, he changed his mind. “I started to think things over in a big way,’’ he said. “Of course it wasn’t worth it.”

Evelina Riabenko contributed reporting.