Beirut — Renewed airstrikes killed at least 20 civilians as they fled government advances in Syria’s besieged eastern part of Aleppo Tuesday, opposition groups reported as troops continued to target the enclave controlled by rebel groups. The United Nations said up to 16,000 people have already been displaced in recent days of fighting.

A senior military official in Damascus put the number of people fleeing the opposition-controlled area Tuesday alone at around 20,000, saying his government has put security measures in place to vet those leaving to ensure no fighters are among them and no violence takes place.

The official, speaking to The Associated Press on condition of anonymity in line with regulations, denied any arbitrary detentions or revenge attacks took place during the influx of displaced, dismissing such reports as a distraction from his troops’ swift success in penetrating rebel defenses.

The U.N. estimates that 275,000 residents are trapped in the area that has been under tight siege and heavy bombardment since July. Many residents and rights groups have expressed fear of revenge or retaliation attacks during the dramatic government advances that have cleaved the territory held by opposition fighters since 2012.

France called for an urgent Security Council meeting to discuss the situation in Aleppo, where ferocious fighting continued as government forces closed in and sought to break down rebel defenses in the deeply divided city, once Syria’s thriving commercial center.

Government forces pushed their way into a new neighborhood on the edge of the rebel-held areas to the east, near the Aleppo airport road, according to the opposition Syrian Observatory for Human Rights. The Britain-based Observatory and government allied media reported pro-government troops were also battling opposition fighters on the western edge of government-controlled Aleppo, seeking to secure the areas and repel opposition fighters from them.

The Russian Ministry of Defense said Tuesday that “half” of eastern Aleppo has been “liberated.”

The Syrian Civil Defense, a team of first responders also known as the White Helmets, said airstrikes hit a group of civilians fleeing to Bab al-Nairab district from government advances in the north, killing 25.

The activist-operated Halab News Network posted footage of the aftermath of the airstrikes, saying at least 20 from three different families were killed. The footage showed body parts strewn along a road full of debris, and families grieving over the corpses of their relatives as rescue workers attended to them. At one point, a warplane roamed overhead.

The Observatory put the death toll at 10, but said it was likely to rise, as dozens were wounded and missing.

In swift and dramatic advances, Syrian government and allied troops pressed their way into northern parts of opposition-held eastern Aleppo in the last couple of days, setting off a wave of panic and flight from the besieged enclave. Many of the fleeing civilians headed to government and Kurdish-controlled areas while others were driven deeper into the remaining rebel-held zones.

The U.N. Humanitarian Chief Stephen O’Brien said he was “extremely concerned” about the fate of civilians, saying new displacements are likely. He called the situation in Aleppo “deeply alarming and chilling.”

“There are no functioning hospitals left, and official food stocks are practically finished in eastern Aleppo,” O’Brien said in a statement. “It is likely that thousands more will have no choice but to flee should fighting continue to spread and intensify over the coming days.”

Residents of the besieged, eastern part of Aleppo reached by telephone reported thousands of civilians have moved to its southern district, cramming into alleyways and empty or deserted buildings.

Farida, a gynecologist who goes by her first name only for fear of reprisals by the government, said the situation has become “unbearable” in eastern Aleppo. “Many people are escaping to government areas,” she said, rushing back to her patient in labor, in one of the few functioning clinics in the enclave.

Abdulkafi Alhamdo, a teacher who has remained in rebel-held Aleppo, said many of the displaced have squatted in his nearly empty apartment building. Alhamdo said 11 empty apartments in his building were already occupied by the newly displaced, many of them women.

“They are not safer from airstrikes, but are at least seeking refuge from the government gangs,” said Alhamdo. He said many of the fleeing civilians fear revenge attacks, amid reports of arrests from people living in government-held areas.

“Those (fleeing) have escaped death miraculously,” he told AP from Aleppo, where airstrikes and shelling continued. “They called it a death trip.”

Amnesty International said it has received reports of Syrian security forces detaining men in Aleppo areas recaptured by the government. The London-based human rights group warned of the potential for revenge attacks, arbitrary detention and torture against people formerly living under opposition control. The reports could not be independently confirmed.

The senior military official said the “human flood” of the displaced from eastern Aleppo requires organization and precision to provide security those fleeing and also to ensure no “sleeper cells” or suicide attackers were among them. He said the government is currently assessing where to relocate the thousands of displaced, including in areas recently captured from the rebels.

The U.N.’s O’Brien said civilians in western government-held Aleppo have also come under indiscriminate shelling by opposition groups — violence that has displaced about 20,000 people in recent weeks.

“The parties to the conflict in Syria have shown time and again that they are willing to take any action to secure military advantage even if it means killing, maiming or starving civilians into submission in the process,” O’Brien said.

No aid has gone into eastern districts of Aleppo since July, while punishing aerial bombings have been hitting hospitals and infrastructure. A brief breach by the rebels lasted nearly two weeks amid fighting around the area.

On Tuesday, Baraa al-Shami, a spokesman for the opposition fighting group al-Jabha al-Shamia, said opposition fighters are clashing with government forces on the edge of the Sakhour neighborhood from which they were repelled Monday.

“The rebels are prepared and they have organized new defense lines because of the new situation on the ground. Military work is primarily defense but we are working on offensives,” al-Shami said. He added that there was intense government bombing of nearby al-Shaar neighborhood, which houses a number of the now defunct Aleppo hospitals.

France’s Foreign Minister Jean-Marc Ayrault called for an urgent U.N. Security Council meeting to try to stop the fighting in Aleppo and bring in humanitarian aid.

France has supported Syrian opposition groups resisting Assad’s Russian-backed forces.

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