Sky News had rare access to a field hospital behind Ukraine's front line. What we witnessed among the doctors and ambulance crews gave clues about the state of the war.

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It is not going as well as Ukraine had hoped. Its president said as much this week. Ukraine's counteroffensive is becoming bogged down, stalling in the face of a well dug in enemy and huge numbers of mines.

"Everything here is mined," Eugen, a doctor, told us.

"Most of these injuries are caused by artillery shelling, strikes and minefields our guys have to walk through."

The counteroffensive started using NATO supplied armour, we were told. But anti-tank mines have blocked their advance, corroborating what British intelligence reported this week.

Fighting's bloody toll

Now the fighting is mostly on foot. Infantry warfare, going trench to trench, and through fields thick with mines.

"When the infantry advances, it's completely different," said Eugen. "They walk on the ground with their feet where there are mines, and we get a lot of patients with amputations and with shrapnel injuries."

That kind of fighting exacts a bloody toll. A soldier arrived from the front, his back peppered with shrapnel. We watched doctors pluck out pieces the size of golf balls.

Another had taken a hit from a mortar.

Doctors worked with infinite care to save his ligaments in an injury deep into his leg. The patient was a medic himself.

Vasyl told us he had only been on the front three days when the Russians attacked.

"It was a mortar attack," he said. "We had just arrived and had to go to our position. We're an assault brigade.

"We were supposed to go to the position, and drones started flying and bombing us."

What was he thinking, we asked.

"That I wanted to get back home. To do my job and go back home."

Then a red case arrived, as they call a patient with life-threatening injuries. Caught in an explosion, Anatoly was badly burned over half of his body.

"The condition of the patient is serious," the doctor told us, "taking into account thermal burns - head, upper limbs, back completely, lower limbs. Well, the condition is critical."

Will he be OK, we asked?

"Well, we always hope."

He needed urgent intervention. After the vital work of stabilising him, he was transferred to an ambulance.

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Aid group providing ambulances 'needs more support'

The dangerous work of racing soldiers from behind the front line to military hospitals is not done by the army. A fleet of ambulances run by the aid organisation MOAS fills the role.

In a previous life it rescued refugees at sea in the Mediterranean and Aegean but has now switched fully to Ukraine.

Its ambulances are fast and small, ideal for dodging the dangers of war, customised with life-saving equipment.

Its founder is American entrepreneur and humanitarian Chris Catrambone, who has sunk millions of dollars of his own money into its work and persuaded others like him to follow. MOAS has stayed low profile during this conflict, but need more support now and gave Sky News exclusive access to its work.

Mr Catrambone said: "As the war goes on and on the needs are going to become more and more and that means qualified medical personnel equipment and things are wearing out.

"It's moving. Things are needed to keep up the pace."

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The work is perilous but essential. The MOAS ambulance carrying Anatoly finally reaches its destination, a military hospital with a specialist burns unit. It gives him the best chances of survival, we are told.

Ukraine does not reveal casualty figures and access to the front line is tightly restricted. But in a day's filming behind the lines, we had a sense of what its soldiers are going through.

What is clear is the counteroffensive is not sweeping through Russian lines. It is heavy going.

A breakthrough cannot be taken for granted. What seems certain is a long hot summer of nasty warfare in trenches and minefields.