Aleppo (Syria) (AFP) - The UN has condemned the "descent into hell" being endured by civilians in Aleppo, with the Red Cross saying nearly 20,000 people have fled a Syrian government offensive on the city in three days.

Diplomats said the UN Security Council would hold an emergency meeting Wednesday in New York on the humanitarian crisis in Aleppo, where the army has captured a third of opposition-controlled east in recent days.

The fighting has prompted an exodus of terrified civilians, many fleeing empty-handed into remaining rebel-held territory, or crossing into government-controlled west Aleppo or Kurdish districts.

Up to 20,000 people have fled the regime offensive in the past 72 hours, the International Committee of the Red Cross said.

ICRC spokeswoman Krista Armstrong said the figure was an estimate and the situation remained fluid as "people are fleeing in different directions".

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

World Food Programme spokeswoman Bettina Luescher said civilians were enduring a "slow motion descent into hell".

The French UN ambassador Francois Delattre said "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".

Government forces have advanced swiftly in their two-week operation, capturing all of the city's northeast in a major blow to the opposition.

- Sleeping in streets -

The loss of their east Aleppo stronghold would be the worst defeat for rebels since Syria's conflict erupted more than five years ago.

The opposition has steadily lost territory in recent months to government forces bolstered by a Russian military intervention since September 2015.

Moscow says it is not involved in the Aleppo offensive, but a Russian defence ministry spokesman said Syrian government forces had seized "nearly half the territory occupied by rebels in east Aleppo in recent years".

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"The careful and long-planned operations by the Syrian army have radically changed the situation over the past 24 hours," said General Igor Konashenkov.

Russian President Vladimir Putin instructed his government to set up mobile field hospitals around Aleppo, the Kremlin said.

A hospital able to serve up to 250 patients a day will be sent to the Aleppo region on Wednesday morning, reports in Russia said.

More than 250 civilians have been killed in the government's assault on east Aleppo since November 15, including nearly 30 children, said the Syrian Observatory for Human Rights.

The monitor said at least 10 civilians were killed in a strike in the Bab al-Nayrab district Tuesday and reported ongoing clashes in the Shaar and Tariq al-Bab neighbourhoods.

The Observatory said the civilian exodus continued Tuesday from neighbourhoods now on the front line.

An AFP correspondent said families were forced to sleep in the streets or in unfurnished apartments left empty by fleeing residents.

Rights group Amnesty International urged Syrian authorities to protect civilians in recaptured areas.

"Given the Syrian government's long and dark history of arbitrary detention and enforced disappearances on a mass scale, it is even more crucial that civilians are protected in newly captured areas of Aleppo city," said Samah Hadid, Amnesty's deputy director for campaigns in Beirut.

- Children 'sitting targets' -

Save the Children warned the assault was separating families and leaving thousands, including children, homeless and at risk.

"With so many people trapped in an ever-shrinking space, children can be little more than sitting targets for bombs," said Syria director Sonia Khush.

On the ground, residents expressed despair and uncertainty for the future, after months of food shortages and heavy bombardment.

Syria's conflict has killed more than 300,000 people and drawn in world powers including a US-led air coalition fighting the Islamic State group.

After a six-week probe, the Pentagon said intelligence errors resulted in a coalition strike that reportedly killed about 90 Syrian government forces in eastern Deir Ezzor in September.

Also Tuesday, at least 1,200 including rebels and their relatives were evacuated from a rebel stronghold near Damascus to the northwest province of Idlib, the Observatory and aid workers said.

Leaders in Khan al-Shih struck a deal with Damascus that would see their town brought back under regime control in exchange for an end to a government siege.

France and Britain, meanwhile, said they would submit to the UN Security Council a resolution for sanctions against Damascus for using chemical weapons.

A joint investigation by the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW) found regime forces used toxic weapons in 2014 and 2015.

And the Turkish army said two of its soldiers were missing in Syria, where it is fighting IS, after the Amaq news agency affiliated with the jihadist group claimed it had taken the pair hostage.