When Syrian troops entered the family's al-Firdous neighbourhood a week ago, they tried to escape the fighting. "They did not allow us to leave - we were all taken to an old cotton factory in the Jibreen area of southeast Aleppo. Men were separated from women and everyone was questioned, and after a few days were allowed to go," he said.
But as the family tried to pass through a checkpoint in the Ramousseh district last Friday, secret service officials checked Mohammed's ID against a list and arrested him on the spot.
"They took his phone and all his belongings. The names on the list were of NGO workers, medics and anyone thought to be aiding the rebel cause. They told my brother, 'We have a situation and you need to help us fix it'.
"I did not speak out, I could not. I knew there was nothing I could say to stop them," said Yussef, a 36-year-old factory worker who was not on the Government's blacklist. "I could only think of my own children and wife and did not want to be detained or killed myself."
The Syrian Observatory for Human Rights, a UK-based monitoring group, estimates that more than 300 people have gone missing from east Aleppo since the regime began its ground offensive last month.
Yussef said he knew of many others who have suffered the same fate and feared there were likely hundreds more than reported.
The Daily Telegraph spoke to two other families who confirmed the detentions. One father, whose son was arrested 10 days ago, had heard his son was already fighting with the Syrian military in the eastern city of Deir Ezzor. The army has been looking to bolster its dwindling numbers, having suffered a huge loss of manpower during the bloody five-year conflict.
"We haven't heard anything from him since December 1," Yussef said. "I think that we will never hear from him again."
Fares Shehabi, an MP for Aleppo, denied civilians were being held, saying they had been offered shelter in the regime-held western side of the city. "All civilians leaving the east are being taken care of by the Government and various civil society groups," he said. "None have been detained to my knowledge."
Since the army swept through the northern part of the rebel enclave a week ago, capturing several large, populous districts, at least 40,000 people have fled across the frontlines from the opposition areas. Thousands more have been displaced and have retreated further into areas still under rebel control, where the situation is becoming more dire by the day.
President Bashar al-Assad's forces, supported on the ground by Russian, Iranian and Lebanese Hizbollah fighters, have regained nearly two-thirds of the east.
The parts still under rebel control have been bombed relentlessly by the regime, which is hoping to empty out the east and reclaim full control of Syria's second city.
A defeat for the rebels in Aleppo - one of their last remaining urban bastions - would be their most devastating loss yet.
Delegates from the United States and Russia were due to meet overnight in Geneva to discuss a deal which could see the withdrawal of all rebel fighters from the city.
The opposition remains defiant, however, telling Washington they would not pull out despite international concern for the remaining civilians.
Abu Abdel Rahman Al-Hamawi of the Army of Islam group said rebels "would fight until the last drop of blood".