Aleppo's doctors and nurses – an ever-dwindling, life-saving workforce operating in harrowing conditions in a city under a years-long siege – don't work in shifts. Instead, they remain in their medical facilities full-time, working, eating and sleeping there.

That way, they can respond immediately when there is an attack.

And there is always another attack.

About 95 per cent of medical staff in Syria's largest city, Aleppo, have been killed, detained or have fled, a new report from Physicians for Human Rights has found, leaving an enormous workload for the 80-90 left behind.

"They live and work in their hospitals for weeks at a time and perform operations for which they had no formal training," the report found.

"These doctors and their fellow nurses, medics, and other health workers remain in the city out of duty; they understand that their departure would result in further loss of lives and decreased access to health care for the 300,000 residents remaining in Aleppo."

The group has documented the deaths of 687 medical personnel and 329 attacks on medical facilities since the Syrian war began in February 2011, and the Syrian regime "is responsible for nearly 90 per cent of these attacks," it found.

"Each attack, whether the bombing of a hospital or the detention and torture of a doctor for providing health care, is a war crime," the group states.

As of August 2015, there were just 10 hospitals functioning in Aleppo, including seven field hospitals for trauma surgery, an ophthalmology hospital, an obstetrics and gynaecology hospital, and a children's hospital, way down from the 33 that serviced the city in 2010, the group found.

With a pre-war population of over three million, Aleppo was the commercial hub of Syria, but in the last five years hundreds of thousands have fled, leaving whole swathes deserted.

Since Russia began its military intervention in Syria on September 30, launching air strikes mostly against areas held by the Syrian opposition forces fighting overthrow the Assad regime, there has been another exodus of civilians from Aleppo.

Humanitarian groups say repeated Russian air strikes, backed by Syrian Army ground troops, had resulted in the displacement of an additional 100,000 Syrians from Aleppo's southern countryside.

In the last month, Physicians for Human Rights documented 10 Russian attacks on Syrian health care centres.

Doctors interviewed by PHR described the fear of living with the unimaginable devastation of barrel bombs, dropped several times a day by the Syrian Air Force, describing attacks in which hospitals were struck time and time again.

"With such frequent attacks, physicians in Aleppo have no doubt that the government is targeting their hospitals," the report said.

"Barrel bombs shred limbs and inflict head to toe injuries on anyone in their large blast radius. Aleppo's skeletal health care system is no match for an influx of patients, each with numerous horrific injuries, and many residents are dying from a lack of medical care," PHR says.

The group reserves some of its harshest criticism – beyond the governments of Syria and Russia – for the United Nations and the international community.

"The UN Security Council … passed Resolution 2139 in February 2014, demanding an end to attacks on civilians and specifically medical spaces," the group says.

"But in the 18 months since the resolution was passed, the Security Council has watched silently as its life-affirming words became hollow and hundreds more medical workers lost their lives at the hands of their government."

Russia's Foreign Minister Sergey Lavrov described as "unsubstantiated accusations" claims that its airstrikes had violated the rules of war, the New York Times reported.