At least 67 of the victims were executed or "unlawfully killed" in the operation in the Latakia governorate and about 200 hostages taken, the organization said in a Thursday release.
The report, "'You Can Still See Their Blood': Executions, Indiscriminate Shootings, and Hostage Taking by Opposition Forces in Latakia Countryside," said two opposition groups participating in the offensive, the Islamic State of Iraq and Sham and Jaish al-Muhajireen wal-Ansar, still held the hostages, the majority women and children.
The findings strongly suggest the killings, hostage-taking and other abuses in the offensive that began Aug. 4 rise to the level of war crimes and crimes against humanity, Human Rights Watch said.
"These abuses were not the actions of rogue fighters," said Joe Stork, acting Middle East director at Human Rights Watch. "This operation was a coordinated, planned attack on the civilian population in these Alawite villages."
To provide victims some justice, the U.N. Security Council should immediately refer Syria to the International Criminal Court, Human Rights Watch said.
Human Rights Watch previously documented war crimes and crimes against humanity by government forces or other forces loyal to President Bashar Assad.
HRW said it found that at least 20 armed opposition groups participated in the operation they alternately termed the "campaign of the descendants of Aisha, the mother of believers," the "Barouda offensive," or the "operation to liberate the coast," which lasted until Aug. 18.
The organization said it wasn't clear whether all or most of those groups were in the villages Aug. 4 when most of the abuses apparently occurred but five groups that were the key fundraisers, organizers and architects of the attacks were "clearly" present from the outset of the operation: Ahrar al-Sham, Islamic State of Iraq and Sham, Jabhat al-Nusra, Jaish al-Muhajireen wal-Ansar, and Suquor al-Izz.
Human Rights Watch concluded through interviews, an on-site investigation and a review of opposition statements and videos that those five armed groups are responsible for specific incidents that amount to war crimes.
Among its recommendations, HRW said the U.N. Security Council should impose an arms embargo on groups if there is credible evidence of widespread or systematic abuses or crimes against humanity.
"Syrian victims of war crimes and crimes against humanity have waited too long for the Security Council to send a clear message that those responsible for horrible abuses will be held to account," Stork said. "The ICC referral is long overdue."
The rights organization also recommended that governments with influence over the armed opposition groups should press them to end their deliberate and indiscriminate attacks on civilians, and called for an end of support, both financially and with weapons.
Also, governments should not permit their countries be used for shipment of arms, ammunition, and other materiel to these groups, Human Rights Watch said.