A video posted on the internet by opposition activists shows bodies lined alongside what they said was the Queiq River in the rebel-held Bustan al-Qasr neighborhood of Aleppo.
The Queiq River rises in Turkey and travels through government-held districts of Aleppo before it reaches Bustan al-Qasr.
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Some 66 people have been killed after the plane they were travelling on crashed in southern Iran.
The letter calls for an end to harassment, abuse, and impunity in a world "ripe for change".
Police say that anyone who sees Whitehouse should not approach him, but instead call them on 999 and quote incident 23 of 18 February.