Sidon (Lebanon) (AFP) - Nine-month-old Amena al-Helou's skin sizzles as the surgeon cauterises an incision in her chest, beginning a heart operation at a south Lebanon hospital to save the Syrian refugee's life.

She is just one of dozens of refugees treated each year by Lebanon's leading paediatric heart surgeon Issam al-Rassi, who each week sets aside a day to operate on Syrian and Palestinian refugees.

But for all his efforts, including on occasion waiving his fee, Rassi's work runs up against the reality that many refugees simply cannot afford the life-saving treatment Amena is receiving.

"I have lost babies while the father was looking for help for money," he says in his office at the Hammoud hospital in Sidon.

"I have a baby who should have been operated on at six months being operated on at nine months because the father needed three months to get the money."

So, despite being born without a right ventricle, Amena is in some ways lucky, because her parents have been able to scrape together loans to pay for her treatment.

She is barely visible as Rassi, 50, and his team perform the procedure, her tiny figure dwarfed by the operating table and cloaked in green sheets.

The skin on her torso is painted brown with antiseptic and wrinkles like old leather as it is pulled apart to reveal her rib cage, which Rassi snips open.

He works to reroute the blood flow from Amena's head directly to her lungs, ensuring it is oxygenated despite the missing ventricle.

The room is quiet except for the occasional request for a tool and the beeping of a machine monitoring Amena's vital signs.

As he completes the procedure, Rassi observes her blood oxygen saturation rate rise to 98 percent.

"It's working," he says.

- 'Huge sums' -

Outside the operating room, Amena's parents Khalil and Amira al-Helou are waiting anxiously to hear the fate of the youngest of their six children.

They have been refugees in Lebanon since they fled their home in war-torn Syria's northeast in 2013, with 39-year-old Khalil relying on seasonal farm work to scrape together money for food.

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Hammoud hospital offers discounts to refugees, and the UN covered 75 percent of her operation -- but the remainder was still nearly $2,000 (1,800 euros), far beyond the Helous' means.

"I gathered the money from different people, my brother, my cousin, other relatives," Khalil said.

"What's hard is not now, but paying it all back later. I don't know how we'll do it."

Khalil said he approached several charities in Lebanon for help but was told they "don't help Syrians."

More than a million Syrians have sought refuge in Lebanon since the war began in March 2011, testing the already-limited resources of the tiny nation and the patience of its four million citizens.

The UN says it lacks the funds to respond to the crisis, meaning refugees who can barely afford food are left to pay thousands of dollars for life-saving operations.

The situation infuriates Rassi, whose voice raises in a rare show of emotion as he discusses it.

"We cannot ask a father living in a tent... to pay $3,000, it's a huge sum of money," he says.

"For a Lebanese middle-class person, it's the salary for three months."

Until recently, Rassi was Lebanon's only paediatric cardiac surgeon and he sees his work with refugees as a matter of responsibility.

"It's really part of our duties, not part of our job. You have a baby, you have to operate on the baby," he says.

- 'Rationing care' -

After Amena's successful operation, Rassi immediately moves on to 18-month-old Ali, who breathes in rasping gasps, eyes buried in his swollen face.

Like Amena, he is missing a ventricle, but his procedure has been delayed by three months.

"The operation should have been done a long time ago, we're doing it late because we had to collect the money," says his 29-year-old father Ahmed Hassoun, from northwestern Idlib province.

While Ahmed borrowed from relatives, Ali developed a chest infection, further delaying the operation.

UN refugee agency UNHCR says it sought $134 million for hospital care for Syrian refugees in Lebanon this year but only received pledges worth $36 million.

UNHCR senior public health officer Michael Woodman, himself a doctor, said that was a huge sum but fell far short of needs.

"We need a much bigger commitment... The fact that we are 30 percent funded for lifesaving care is appalling," he told AFP.

The UN is currently only able to cover 50 percent of the health needs of Syrian refugees in Lebanon, he said.

"It's very difficult, it's a tragedy... No one wants to be in this position of rationing health care, we shouldn't be."

For now, Amena's parents are focused on her recovery, her mother tearing up at the sight of her fragile daughter in the ICU, sedated and breathing through a tube.

"Of course you feel afraid," Khalil says quietly, "but there's no other option."

"She's the most precious thing I have."