Images shared on social media have illustrated the anger that Syrian rebels fighting President Bashar al-Assad feel toward his ally Vladimir Putin. The Russian leader has been indispensable in propping up Assad's regime during the brutal Syrian civil war that started in 2011 but has been essentially frozen over the last four years.

Anti-government forces have taken control of Syria's second city Aleppo. But who are the rebels and what are their goals? One key player is the group Hayat Tahrir al-Sham - or HTS, which already controls much of northwest Syria and is expanding its reach.