In the early days of the invasion, Kharkiv in eastern Ukraine fought back a Russian armoured column. Since then, it has suffered nightly Russian airstrikes and shelling, with dozens of civilians killed and hundreds injured. The BBC's Quentin Sommerville and cameraman Darren Conway have spent the week with the Ukrainian forces as they fight to stop a further Russian advance.

This report contains material some viewers will find disturbing

The first casualty of war is time. Ask the young soldier at the front when the attack happened, or the old lady in the hospital bed when her home was shelled, and they look at you confused. Was it 24 hours ago, or 48? The days have become one, they tell you.

In Kharkiv, Ukraine's second-largest city, time is elastic. It's close to the border with Russia and the nightly shelling from Russian artillery and warplanes gives no rest. The past two weeks have seemed like an eternity, yet peace can be remembered as if it were yesterday.

In a frozen landscape on the city's north-eastern edge, 21-year-old Lieutenant Yevgen Gromadsky stands with hands outstretched. There are trenches dug in nearby. "Outgoing," he says, lifting his right hand to accompany the thump of fire from his positions. "Incoming," he says, and his left hand ticks up. With a crump, Russian shells are fired from their positions 900m away across snow-covered fields.

The shelling continues like clockwork at the edge of this bombed-out village - "Incoming, outgoing, incoming, outgoing," Lt Gromadsky flicks his hands with each report.

We met only this afternoon, but already I know that just last week, his father Oleg was killed defending the city, and Lt Gromadsky is the seventh generation of military in his family. He plans for an eighth, in a free Ukraine.

He describes the fight so far, "Sabotage groups are probing our lines out, we have direct tank battles. They shoot with mortar shells at first, and then tanks fire at our positions."