How Ukraine is revamping Soviet-era weapons for a 21st-century battlefield

Along the eastern front line, in command centers in basements hidden behind metal doors, Ukrainian soldiers direct artillery fire in an attempt to impede the Russian advance.

Their fiercest fight is taking place for the city of Bakhmut, which has been besieged by Russian forces for months. The ferocity of that battle is evident from the first moments of approaching the city, where black smoke emanates from the apartment blocks.

As a CNN crew drove into the main street, a Russian shell hit a building just a few dozen meters away. Moments later, another shell hit the building again.

The Kremlin has concentrated a large number of forces on this attack on Bakhmut, and Ukrainian troops are fighting back, says Petro, the National Guard commander who heads the unit.

"It feels like a constant, non-stop attack," he says. "The only window to rest is when they run out of people and wait for reinforcements."

Like others in the Ukrainian military, Petro uses only his first name to protect his identity.

It describes a battle in which Russia has sent wave after wave of forces, caring little whether they were mowed down.

"Their tactics are sending these poor people forward that we have to eliminate," explains Petro. "They can't take Bakhmut with a direct attack, so they surrounded him. We had to move from the urban areas to the fields where we are very exposed to artillery".

Some Russian soldiers have described significant casualties from Moscow's ranks, although the Russian Defense Ministry earlier this month claimed losses "did not exceed 1% of the combat force and 7% of the wounded".

Ukrainian commanders also complain of a lack of communication between units and that they lack enough lower-level officers to keep soldiers motivated and engaged after months of grueling combat.

Tuman, the battery commander, takes coordinates on a cell phone in one hand and writes them down in a notebook he holds in the other. He claims that the accuracy of Russian artillery has deteriorated.

"Their accuracy dropped," he says. "But their rounds are flying over us all the time."/ CNN