The August 4 attacks on unarmed civilians in more than a dozen villages in the coastal province of Latakia were systematic and could even amount to a crime against humanity, Human Rights Watch (HRW) said in a 105-page report.
The findings are based on a visit a month later to the area, with permission from the Syrian regime.
The report quoted witnesses as saying rebels went house to house, killing entire families or killing the men and taking the women and children hostage.
The villagers belong to the minority Alawite sect, an offshoot of Shi’ite Islam which forms the backbone of President Bashar Assad’s regime — and which Sunni Muslim extremists consider heretics.
One survivor, Hassan Shebli, told HRW he fled as rebels approached his village of Barouda at dawn, but was forced to leave behind his wife, who was unable to walk without crutches, and his paralysed 23-year-old son.
When Shebli returned days later, after government forces retook the village, he found his wife and son buried near the house and bullet holes and blood splatter in the bedroom, the New York- based group said.
The findings are bound to feed mounting Western unease about the tactics of some of those trying to topple Assad and about the growing role of jihadi rebels, including foreign fighters linked to al-Qaida.
The main Western- backed rebel alliance, the Free Syria Army, distanced itself from the five rebel groups named by the HRW as the main perpetrators.
“Anyone who commits such crimes will not belong to the revolution anymore,” said spokesman Louay Mikdad.
He said the alliance is not cooperating with extremist groups, and that al-Qaida-linked rebels frequently attack FSA fighters.
A rebel in the Latakia area, who goes by the name of Mohammed Haffawi, denied civilians were killed during the offensive. Reached by Skype, he said one of the rebel groups is holding about 100 women and children as bargaining chips for the release of prisoners held by the regime.
UN war crimes investigators have accused both sides in Syria’s civil war, now in its third year, of wrongdoing, though they said earlier this year that the scale and intensity of rebel abuses hasn’t reached that of the regime.
The new allegations of rebel abuses come at a time when the regime is regaining some international legitimacy because of its apparent cooperation with an internationally mandated programme to destroy Syria’s chemical weapons stockpile by mid-2014.