Up to 20,000 people have fled eastern Aleppo over the past 72 hours as Syrian regime forces and its allies continued to advance in the opposition-held part of the city, according to the Red Cross.

Situation is Aleppo is a ‘human catastrophe’

Terrified civilians have fled empty-handed into remaining opposition-held territory, or crossed into regime-controlled western Aleppo or Kurdish-held districts.

The 20,000 figure is an estimate and could increase as "people are fleeing in different directions", International Committee of the Red Cross spokesperson Krista Armstrong told the AFP news agency.

United Nations humanitarian chief Stephen O'Brien had earlier put the number of displaced people from eastern Aleppo at 16,000.

The city, which was Syria's biggest before the start of the war that has killed hundreds of thousands of people, has been divided between the regime-held west and opposition-held east, where UN officials say at least 250,000 people remain under siege.

The Syrian regime offensive to recapture the opposition-held parts of Aleppo has sparked international alarm as it intensified this week.

A voluntary rescue group known as the White Helmets reported at least 51 civilians killed in east Aleppo and more than 150 injured during the regime assault.

Syrian regime forces dropped "more than 150 air strikes from war planes and helicopters and [fired] more than 1,200 artillery shells", the group wrote on its Facebook page.

The attacks hit the neighborhoods of Bab al-Neirab, al-Mayser and al-Salheen, among others.

France called for an immediate UN Security Council session on the fighting, which has seen the army capture a third of opposition-controlled east Aleppo in recent days.

The UN Security Council will hold an emergency meeting Wednesday on the dire humanitarian crisis unfolding in Aleppo, diplomats said.

The 15 ambassadors of the UN Security Council will get a video conference briefing on the situation in Aleppo by a UN official in charge of humanitarian operation and the UN mediator in Syria, Staffan de Mistura.

"France and its partners cannot remain silent in the face of what could be one of the biggest massacres of civilian population since World War II," said France's UN ambassador Francois Delattre on Tuesday.

He and his British counterpart Matthew Rycroft earlier in the day pushed for the emergency council meeting on providing humanitarian relief to the besieged Syrian city.

Eastern Aleppo has been under regime siege for more than four months, with international aid stocks exhausted and food supplies running low.

Rycroft said the council would discuss plans for the UN to deliver much-needed food and medicine into Aleppo and evacuate the sick and wounded.

The Syrian conflict started as a largely unarmed uprising against Bashar al-Assad's rule in March 2011. It has since morphed into a full-on war that has killed hundreds of thousands.

The UN refugee agency has registered more than 4.8 million Syrian refugees who have fled the fighting, while another 6.1 million people are internally displaced within the country's borders.

PHOTO CAPTION

Civil defense members and men inspect a site damaged after an airstrike in the besieged opposition-held al-Qaterji neighborhood of Aleppo, Syria October 11, 2016. REUTERS

Al-Jazeera