"I've cried a lot in the past nine months," says Dr Kassem al-Zein, founder of a field hospital in the besieged rebel-held Syrian city of Qusayr, "especially when a child dies in my arms."

With three children of his own, Zein chokes up as he tells of five youngsters brought in two months ago, all hit by shrapnel from one of the countless mortar bombs that rain down on the city along with Grad rockets and other artillery.

"They all had serious head or chest wounds, and some were dead on arrival. I did everything I could but, in the end, all five died."

Qusayr was once a city of 50,000, but its population has shrunk to around 10,000 as people have fled daily shelling and house-to-house fighting in President Bashar al-Assad's campaign to retake it.

The hospital, located in an old house in the city centre, is no exception.

"We are a main target for the regime, which is why they bombard us constantly," Zein says.

There is a huge hole in the building's facade, the result of a direct hit, and the staff know that each day could be their last.

"All of us are afraid because we are being hunted by the regime, and if they catch us, they will execute us as traitors.

"They execute us because we are doing our job -- saving lives. We have become enemies."

The hospital was set up nine months ago by doctors who escaped from the city's National Hospital, which is under the control of regime forces.

"We admitted around 570 patients this month wounded by the bombs that constantly fall on the city and another 100 with gunshot wounds, mostly from snipers," Zein says.

A loud honking outside interrupts the conversation; the morning's first wounded are arriving.

A man rushes in carrying an 11-year-old girl in his arms, leaving an intermittent trail of blood in the corridor behind him, his own shirt soaked red.

The girl is dead, her head destroyed by shrapnel.

A few seconds later, and more honking. Nurses quickly remove two boys, one aged three the other four, from a car and rush them inside.

Their faces are covered with blood, but Dr Saleh Mahmud Sadir says they'll be fine.

"They only have a few cuts on their heads and extremities," he says.

"Now do you understand why we stay here? We stay for them, the people."

Sadir has his own grief to bear. His son died in his arms after taking a direct hit.

"My duty and my mission is to keep working until the regime falls. I became a doctor to help people and save lives. We are on the right side."

Umm Amaar, one of the nurses, tries to wipe the blood from one of the boy's face as a colleague stitches the wound.

Boom!

Another bomb hits the building, breaking the windows of the emergency room, and the nurses quickly move the wounded further inside. More wounded arrive and have to be put wherever there is space.

"Life always takes precedence over death," chief nurse Rabia Ismail says.

"We give priority to those we think have a possibility of surviving surgery. We can't treat everybody equally because, if we did, we would not manage to save any lives.

"We have to choose who lives and who dies, but at the end of the day we think, no, it wasn't we who killed them, it was Assad's bombs."

Dr Sadir elaborates, saying "we don't have the equipment needed to treat the most serious cases. We don't have a neurosurgeon, so if there are head wounds, the only solution is to send the patients to Lebanon."

"As for the others ... there is little we can do except try to ease their pain and make their deaths as painless as possible. The rest is in the hands of God."

Little by little, the chaos subsides as the shelling eases off, and the frayed nerves begin to relax.

But Ismail recalls what he says was probably the most frightening day of his life, a day when more than 50 wounded were brought in as shelling continued outside.

"One bomb hit just 15 metres (yards) from the hospital, killing a man who was coming to donate blood. But despite everything we continued stitching the wounds, even as we risked being hit."