Civilians of Syria's east Aleppo facing 'savagery' amid army advance: UN

Civilians under bombardment in Syria's rebel-held east Aleppo are facing "a level of savagery that no human should have to endure," the UN aid chief said Sunday.

Stephen O'Brien, who heads the United Nations humanitarian office (OCHA), issued a fresh plea to ease the suffering of some 250,000 people besieged by a Russian-backed Syrian government offensive to retake the key city. 

In a statement, O'Brien called for "urgent action to bring an end to their living hell".

Syrian President Bashar al-Assad is battling to reclaim Aleppo, once the country's economic powerhouse. Diplomatic efforts to stem the bloodshed have so far failed. 

Fierce bombardment overnight on Saturday helped the Syrian ground forces continue their advance in the north of the city on Sunday morning.

If loyalist fighters seize Al-Heluk, Bustan al-Basha and Sakhur - all rebel-controlled neighbourhoods in Aleppo city's north - they will confine opposition factions to a small section of the city's southeast.

The advance and accompanying air campaign have sparked international outrage, particularly after a number of strikes appearing to target medical facilities.

"The healthcare system in eastern Aleppo is all but obliterated," O'Brien said, after the largest hospital in the rebel-controlled area was hit by barrel bombs on Saturday. 

"Medical facilities are being hit one by one," he added. 

On Saturday, European Parliament president Martin Schulz called the hospital bombing a "war crime", tweeting that the international community "must unite to prevent (the) city's annihilation".

And French Foreign Minister Jean-Marc Ayrault said "the systematic targeting of structures and health workers is particularly unjustifiable".

O'Brien urged warring parties to at the very least allow medical evacuations for the hundreds of civilians in urgent need of care.

The UN has said that water and food supplies in eastern Aleppo are running low, while efforts to bring in aid convoys through the Turkish border have been stalled by the fighting. 

The UN had hoped it could restock east Aleppo during a ceasefire negotiated last month by the United States and Russia, but security conditions to allow those deliveries were not met and the ceasefire quickly collapsed. 

With many basic medications now unavailable most supplies are running short, patients are being turned away from health centres and the need for evacuations is likely "to rise dramatically in the coming days," O'Brien said. 

The battle for Aleppo has sparked some of the most brutal violence since the beginning in March 2011 of Syria's conflict, which has killed more than 300,000 people and displaced over half the population.