The Mi-17, likely to be based at Mezze, lost both its tailboom and rotors. Indicating the helicopter was hit by a large missile at high altitude, not by a MANPAD or AA fire.
Footage published by the YouTube channel of Jaish al-Islam shows the Mi-17 getting hit by a 9M33 missile fired from Jaish al-Islam's 9K33 Osa. At 0:47, the 9M33's sixteen kilogram explodes just above the tailboom and rotors. Which in turn, explains the loss of these two components.
Another 9K33 Osa launch was uploaded on the 18th of January 2014.
Opposed to the other launches, this one likely missed. The 9M33 missile self-destructs 25-28 seconds after launch. Either this one hit a target at the edge of their range or it went out of control and self-destructed.
Jaish al-Islam using a 9K33 Osa (1)