The bodies were found in two neighbourhoods in east Aleppo, state news agency SANA said late Sunday.
The head of Aleppo's forensic unit Zaher Hajjo told SANA that "21 corpses of civilian victims, including five children and four women, killed by terrorist groups" were examined.
"The bodies were found in prisons run by the terrorist groups in Sukkari and al-Kalasseh, and they were found to have been executed by gunshot at very close range," Hajjo was quoted as saying.
Days before the evacuations began, the UN said it had received credible reports of at least 82 civilians, including 11 women and 13 children, being executed by pro-government forces in Aleppo.
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On Monday, the Russian defence ministry said "dozens of Syrians" were summarily executed in east Aleppo by rebels.
"Mass graves containing dozens of Syrians who were summarily executed and subjected to savage torture have been discovered," spokesman Igor Konachenkov said, according to Russian agencies.
The Syrian Observatory for Human Rights confirmed that bodies had been found in east Aleppo's streets, but could not specify how they had been killed.
World powers have been fiercely divided over Syria's conflict since it first erupted in March 2011, with Russia firmly backing Assad and Gulf powers and much of the West supporting the opposition.
The high-profile battle for Aleppo, in particular, has sparked accusations by Western powers that Russia and the government were committing war crimes.
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