The attack is the latest in a series of incidents that show the increasingly sectarian nature of the conflict in Syria.
The coastal region of Latakia is home to the majority of Syria's Alawites, the minority sect to which President Bashar al-Assad belongs.
Extremist groups within the largely Sunni opposition have been known to target Alawite and Christian areas in retaliation for government attacks against rebel-held territory.
Among the groups involved in the offensive was al-Qaeda offshoot the Islamic State of Iraq and as-Sham and its affiliate Jabhat al-Nusra.
Other Islamist groups such as Ahrar al-Sham, Jaish al-Muhajireen wal-Ansar and Suquor al-Izz were also involved in the operation.
In recent months, clashes between extremist rebels and more moderate groups within the opposition have been on the rise.
The Human Rights Watch report said at least 20 distinct armed opposition groups participated in the operation they termed the "campaign of the descendants of Aisha, the mother of believers", the "Barouda offensive", or the "operation to liberate the coast".
Human Rights Watch has previously documented war crimes and crimes against humanity by Syrian Government and pro-government forces. These include torture and illegal executions.
- Independent