Syrian forces have rounded up hundreds of young men as government troops advance into rebel-held zones in embattled Aleppo, residents and officials said, prompting grave fears for the safety of those held.

Similar widespread detentions - apparently seeking rebel fighters or personnel for forced conscriptions - have followed the fall of other opposition strongholds during the more than five-year 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 in efforts to topple one of the last major urban bases for rebels.

Also yesterday, the White Helmets said at least 45 people were killed in government shelling as they roamed the streets of Jub al-Quba, an eastern neighbourhood, looking for shelter.

More than 50,000 civilians have fled the area since pro-Assad forces - backed by Russian air power - overran a third of the rebel stronghold in recent days. Families remaining in opposition territory said that the phones of sons, fathers and brothers had fallen silent. One man who had fled the Masakan Hanano area said forces had detained young men who were hold up in his brother's home. "My sister said he was separated from the group by thugs. We've tried his phone, we've tried his friend's phone. We've heard nothing." Another man said two male relatives were taken in similar circumstances. "They decided to stay at home, not to leave. Now they are arrested."


Assad's security forces have been accused of summary killings inside their detention facilities.

The Syrian Observatory for Human Rights said more than 300 people were missing from Aleppo. The men were believed to have been taken to a nearby air base for screening and interrogation. Activists said some were expected to be forcibly conscripted, an increasingly common practice as the Syrian Army faces manpower shortages. The army denied making any arrests, saying that men were being put in "specific places," a military official told Reuters.

More than 20,000 people had fled to Aleppo's regime-held western neighbourhoods, according to the Observatory, while another 30,000 had moved to areas held by Kurdish forces. The Syrian Arab Red Crescent said it had provided medical care, blankets and hot meals to at least 1000 of the displaced. But many more were sheltering in the rain in abandoned, shelled-out buildings, as well as schools, mosques and tents.