Turkey on Tuesday said it was "horrified and outraged" by what it said was the massacre of civilians by the Syrian regime and its supporters in eastern Aleppo and that their actions were in serious breach of international humanitarian law.

Rebel defences in Aleppo collapsed on Monday, leading to a broad advance by the Syrian army across more than half of the remaining insurgent pocket in the city and a retreat of opposition fighters to a few districts.

Calling for an immediate halt of attacks and safe evacuation of civilians in eastern Aleppo, the Turkish foreign ministry said in a statement the Syrian regime was giving no chance to civilians wishing to leave Aleppo and that its actions amounted to mass executions.

Turkey added that it was deeply concerned that the tens of thousands of civilians stuck in two neighborhoods in Aleppo, where opposition fighters are also present, could face a similar fate.