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.
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Bono said the death of Dennis Sheehan, who had been the band's manager for more than 30 years, was like "losing a family member".
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Prime Minister plans a whistle-stop charm offensive with EU leaders this week.