Opposition activists have posted a video purporting to show at least 51 muddied male bodies alongside a river bank in the rebel-held Bustan al-Qasr neighbourhood of Aleppo.
The video, which is too graphic to post, follows a man walking along a river bank past dozens of male bodies with their hands tied and blood coming from their heads.
More top news
Work to recover the wreckage of downed Malaysia Airlines flight MH17 has finally finished.
Five political leaders have agreed to tackle young people's questions in a series of live Q&As on YouTube. Here's how you can get involved…
The star says she has a "great relationship" with her ex and it's "nice for the children that they know Mum and Dad are still there."