Syrians fleeing eastern neighborhoods of Aleppo to escape fighting on Nov. 20, 2016. (Uncredited/AP)

Syrian forces have rounded up hundreds of young men as the government pushes its advance into rebel-held zones in the embattled city of Aleppo, residents and a monitoring group said Wednesday, prompting concerns about the safety of those held.

Similar widespread detentions — apparently seeking rebel fighters or personnel for conscription — have followed the fall of other opposition strongholds during the five-year-old conflict in Syria. But the scale of the Aleppo showdown is far bigger, and the government of President Bashar al-Assad has stepped up attacks as it seeks to topple one of the rebels’ last major urban bastions.

Also Wednesday, the White Helmets volunteer rescue group said at least 45 people — many looking for shelter — were killed in government shelling in Jub al-Quba, an eastern neighborhood. In photographs published to social media, rescue workers were seen picking through a street strewn with bodies and suitcases.

More than 50,000 civilians have fled rebel-held eastern Aleppo since pro-Assad forces overran a third of the area in recent days. Families remaining in opposition territory said Wednesday that the phones of sons, fathers and brothers had fallen silent.

One man who had fled the Masaken Hanano area — retaken Saturday by Iraqi and Lebanese Shiite militias allied with the government — said young men holed up in his brother’s home were detained. “My sister said he [the brother] was separated from the group by thugs. We’ve tried his phone, we’ve tried his friend’s phone. We’ve heard nothing,” he said. 

Another man said two male relatives were taken in similar circumstances. “They decided to stay at home, not to leave. Now they are arrested,” he said.

Families interviewed by The Washington Post spoke on the condition of anonymity, fearful of the fate that awaits detained relatives. Assad’s security forces have been accused of summary killings of detainees.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said more than 300 people were missing from Aleppo. The men were thought to have been taken to a nearby air base for screening and interrogation. Activists said some are expected to be forcibly enrolled in the armed forces, an increasingly common practice as the army faces manpower shortages.

The army denied making any arrests, instead saying Wednesday that men were being put in “specific places,” the Reuters news agency reported.

More than 20,000 people had fled to government-held western neighborhoods of Aleppo, according to the Observatory, while 30,000 had moved to areas held by Kurdish forces.

As armed rebel groups cling to what remains of their strongholds in eastern Aleppo, the suffering of tens of thousands of civilians still there — already struggling through a four-month-long siege — has deepened. 

On Wednesday, many families were searching for shelter after fleeing the government’s advance farther north. Residents said most had arrived with nothing, raising fears that dwindling food supplies and medicine stocks — last replenished by aid agencies in July — would run out soon.

The Syrian Arab Red Crescent said Wednesday that it had provided medical care, blankets and hot meals to at least 1,000 of the displaced. But many more were sheltering in the rain in abandoned, shelled-out buildings, as well as schools, mosques and tents.

“The implications of fleeing home are immense. It’s not just a matter of picking a few items and leaving. There’s a huge knock-on effect,” said Marianne Gasser, head of the International Committee of the Red Cross delegation in Syria. “They need to be guaranteed safe passage and be offered dignified conditions. And they need all the basic necessities such as food, safe drinking water and medical care.”

The government-led offensive has brought Assad’s forces closer to the recapture of Aleppo and their greatest victory of the civil war. Defeat of the rebels in this strategic city, once Syria’s economic powerhouse, would be devastating for what remains of the broader fight against his rule.

According to the White Helmets, more than 500 people have died and 1,500 have been wounded in the operation.

The death toll is likely to spiral in the coming days as attacks by government forces target areas where the influx of displaced people has swelled the population. 

Abdulkafi al-Hamdo, an English teacher, described Wednesday’s bombing and shelling in rebel-held areas as relentless. The bombs were landing, he said, “like rain.”