Residents say they have not heard from relatives arrested in Masakan Hanano district after it fell on Sunday to Iraqi and Lebanese militias

Residents of east Aleppo have said they hold grave fears for as many as 500 men who were seized by forces loyal to Bashar al-Assad as they overran opposition strongholds in the city.

Three families contacted by the Guardian said there had been no word from their sons and nephews who had been arrested in the Masakan Hanano district, which fell on Sunday to Iraqi and Lebanese militias within hours of the biggest ground offensive of the war being launched.

“They took my nephew and my uncle, one was 22 and the other 61,” said one man who fled Masakan Hanano. “I don’t know if I’ll ever see them again.”

Recaptured neighbourhoods were handed over to the Syrian military, which detained the men across northern Aleppo. After capitulating as the pro-Assad forces arrived, rebel groups were frantically trying to defend what remained of their heartland in east Aleppo, a broken and twisted corner of Syria’s second city that has come to define their plight nationwide.

The collapse of rebel defences has led some military observers to predict that the fall of the city could take place before the end of the year, a development that would spell the end of the opposition’s aim of reorienting power away from Assad, the Syrian president.

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Russian bombers, which have pulverised east Aleppo for much of the past year, were again active on Tuesday, targeting what remained of neighbourhoods closer to the Old City, which demarcates the rebel east and the regime-held west.

The United Nations said 16,000 people had fled the offensive in recent days, with thousands more likely to join them. Aid agencies say as many as 250,000 people still live in east Aleppo, which over the past year has become one of the most uninhabitable places on earth.

“Intensified ground fighting and indiscriminate aerial bombardment over the past few days in eastern Aleppo city has reportedly killed and injured scores of civilians,” said UN humanitarian chief, Stephen O’Brien. “There are no functioning hospitals left, and official food stocks are practically finished.”

O’Brien said up to 20,000 people had fled the west of the city in recent weeks amid a failed opposition bid to take a corner of Aleppo’s southwest in order to open a supply line to the east.

West Aleppo remains comparatively unscathed, four and a half years after rebels from the north Aleppo countryside first stormed into eastern neighbourhoods, ousting Syrian forces in a day-long rout.

Since then, Aleppo has been one of several centres of gravity in the Syrian civil war, its fortunes central to the fate of the conflict itself. Over the past year, the city’s importance to the regional dynamic has been emphasised by the buildup of large numbers of militias sent to Syria to defend Assad’s battered army, which had been unable to reclaim lost areas under its own steam. Iraqi militias have been critical to the fight, as have Hezbollah forces from Lebanon.

Iraqi analyst Hisham al-Hashimi said as many as 15,000 Shia fighters had been sent from Iraq to Syria, with many deployed around Aleppo.

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Hashimi said the Iraqi fighters have been leading attacks but would not hold areas inside Aleppo. Senior sources connected to Hezbollah confirmed on Monday that the group had played a lead role in the fighting. Up to 1,700 members of the group have been killed fighting in Syria since the start of the nearly six-year war, the sources confirmed.

Forces loyal to Assad continued on Tuesday to maintain a stranglehold around the eastern edge of Aleppo, blocking potential escape routes to the countryside. “To be honest, it is very dangerous to try to leave,” said Hassan Mustafa, a local from the Bustan al-Qasr area. “But our resolve is broken. Their tactics have worked. They have bombed us to our deaths, even while we still breathe.”

A second local, who refused to be identified said: “It is hard to hold the faith. For so long we have dreamed that people were coming to help us. The reality is no one will. We will die in this place. And humanity will die with us.”

European diplomats cautioned that the total recapture of the city could lead to retribution against local communities, which had held out since mid-2012. “I’m not just talking about army-aged men,” said one senior official. “This could well lead to something off the scale, involving people from all walks of life. There is no way to police this, and little means to bring the perpetrators to account.”