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President Bashar al-Assad's forces had gained control of 99 percent of the onetime opposition stronghold of east Aleppo, a military source said, signalling an impending end to the rebels' four-year hold over parts of the city as the final hours of battle played out. 

Militias loyal to Assad's forces reportedly executed dozens after capturing rebel-held neighbourhoods, according to pro-regime site "Aleppo on-Air". The source could not be verified.

"Regime forces backed by Russian warplanes advanced in Bustan al-Qasir, al-Kalasa and al-Furdus, forcing the rebels into retreat," activist Mohammed al-Halabi told °®Âþµº.

UN humanitarian adviser on Syrian Jan Egeland warned on Monday that the Syrian regime and the Russian government must be held accountable for the atrocities committed by pro-regime militias.

"The governments of Syria and Russia are accountable for any and all atrocities that the victorious militias in Aleppo are now committing!" Egeland , as the bombardment of the city besieged continued. 

"The battle in eastern Aleppo should end quickly. [The rebels] don't have much time. They either have to surrender or die," Saeed said. Head of the regime's Aleppo security committee Lieutenant General Zaid al-Saleh said the "operation in eastern neighbourhoods is entering its final phase".

"We're living the final moments before victory," Saleh told reporters in the recaptured Shiekh Saeed district of the city.

Ibrahim al-Haj, from the Syrian Civil Defence, said "the situation is very, very critical".

He said he was seeking shelter for himself and his family, fearing clashes or capture by the government.

Meanwhile, state television showed people celebrating, holding up portraits of Assad and Syrian flags.

"Aleppo residents express their joy at the victory against the terrorists", read a caption beneath the footage, referring to the rebels.

The fall of Aleppo would be the worst rebel defeat since Syria's conflict began in 2011, and leave the government in control of the country's five major cities.

Syria's rebels seized control of east Aleppo in 2012, a year into an uprising that began with anti-government protests but spiralled into a complex multi-front conflict, drawing in proxy powers and militants such as the Islamic State group.