France calls for emergency UN Security Council session as battles rage for control of Syria’s second city.

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

Terrified civilians have fled empty-handed into remaining rebel-held territory, or crossed into government-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 a civil war that has killed hundreds of thousands of people, has been divided between the government-held west and rebel-held east, where UN officials say at least 250,000 people remain under siege.

The Syrian government offensive to recapture the rebel-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 government assault.

Syrian government 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.

READ MORE: I live in Aleppo, under siege

The attacks hit the neighbourhoods of Bab al-Nairab, al-Mayser and al-Salheen, among others.

SANA, the official Syrian state media arm, reported that Syrian government forces and allies on Monday took control of several areas in the city’s northeast, including al-Haidariya, al-Sakhour, al-Inzarat, al-Sheikh Khedr, Jabal Badro, and al-Halk.

‘Cannot remain silent’

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 on Wednesday on the dire humanitarian crisis unfolding in Aleppo, diplomats said.

Syrian Civil Defence says at least 51 people were killed in attacks on eastern Aleppo on Monday [Abdalrhman Ismail/Reuters]

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.

READ MORE: How much longer can east Aleppo hold out?

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 government 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.

“Russia complained that the opposition had not agreed to this plan. Now they have, so I call on Russia to make sure the Syrian regime agrees,” Rycroft said.

“The future of Aleppo is in the hands of the regime and Russia, and we urge the regime and Russia to stop the bombing and let the aid go through.”

The Syrian conflict started as a largely unarmed uprising against President Bashar al-Assad’s rule in March 2011. It has since morphed into a full-on civil 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.