Ukraine-Russia war: On the front line with Ukrainian troops as they try to hold back the Russian tide

You cannot see the Russians from the forest floor, but you certainly hear them in the stands of pine that grow north of the city of Kupyansk.

The Ukrainian President, Volodymr Zelenskyy warned that their soldiers were "concentrating maximum reserves" in area.

It is a challenge that the members of Ukraine's 32nd Brigade, who are charged with defending the territory, would struggle to match.

They took us to a command centre located a few kilometres from the front and we found a series of long faces and hard stares in their cramped, underground cabin.

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"They're storming us, breaching the barbed wire so they can pass right through," said a tired looking soldier called Sasha.

He told us that the Russians used to assault their positions once or twice a day - but now the attacks are continuous.

Members of the unit also spoke of crippling ammunition shortages.

"When Ukraine fires an artillery shell, how many shells do the Russians send back," I asked them.

"One to ten," said one. "Yes, one to ten," said another.

Drones give the brigade a comprehensive view of the forest but sometimes they work to highlight the imbalance.

"They have their aircraft, helicopters, planes, laser bombs, "Grads". They use everything along our front line," said Sasha.

President Zelenskyy is desperate to reset - and rearm - the Ukrainian military and has warned that an "artificial deficit" of weapons is giving Russia the space to attack.

Leading US military officials agree, insisting the tide could turn to Putin's "significant advantage", with $60bn of US funding stalled by supporters of former President Donald Trump in Congress.

Zelenskyy's point was made for him when we found members of an anti-tank unit struggling to make their one and only rocket launcher work.

Screwdrivers and a hammer were deployed to bring the Soviet-era bazooka to life.

The commander, who calls himself 'August', said the weapon's reliability was an issue but the shortage of grenades was a far bigger headache.

"Have you got enough ammunition?" I asked.

"No, not enough. We've got enough for 12,13, 15 minutes (of fighting). That's it."

"And after 12 minutes, then what?" I asked.

"We'll take a machine gun and shoot with that," he responded gloomily.