The Syrian government offensive to recapture rebel-held parts of Aleppo sparked international alarm on Tuesday, with the Red Cross saying nearly 20,000 people had fled the assault in two days. Diplomats said the UN Security Council would hold an emergency meeting by Wednesday on the humanitarian crisis in the city, where the army has captured a third of opposition-controlled east Aleppo 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. With so many people trapped in an ever shrinking space, children can be little more than sitting targets for bombs Sonia Khush, Save the Children Up to 20,000 people have fled the regime offensive in the past 48 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”. Government forces have advanced swiftly in their two-week operation, capturing all of the city’s northeast in a major blow to the opposition. 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”. “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 on Tuesday and reported ongoing clashes in the Shaar and Tariq al-Bab neighbourhoods. The Observatory said the civilian exodus continued on Tuesday from neighbourhoods now on the front line. Amid the horror of Aleppo, Syria’s White Helmets volunteers summon the courage to save lives 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. 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 Islamic State.