HRW said on Friday that many of the dead had been executed by militant groups, some linked to al-Qaeda, who overran army positions at dawn on August 4 and then moved into 10 villages nearby where members of Alawite sect lived.

In its first government-sanctioned trip into Syria during the 2-1/2-year conflict, New York-based HRW has documented a series of sectarian mass killings by Assad's foes during a broader campaign in which Western-backed rebels took part.

In some cases, entire families were executed or gunned down as they fled, according to a report titled "You Can Still See Their Blood".

HRW identified five rebel groups instrumental to funding, organizing, planning and carrying out the Latakia attacks, including the al Qaeda-linked  al-Nusra Front and Islamic State of Iraq and the Levant groups, as well as the extremist group Ahrar al-Sham and another unit of foreign terrorist fighters.

These groups publicized their involvement through videos and statements, some of which were used to corroborate the HRW report. The operation appeared to have been largely financed by private Persian Gulf-based donors, HRW said.

What is less clear is the role of militants from the so-called Free Syrian Army (FSA), the armed wing of the main opposition coalition which is openly supported by the United States, Britain, France and Arab Persian Gulf states.

In a video posted on August 11 and apparently filmed in Latakia, FSA chief Salim Idriss said the body was participating in the offensive "to a great extent."

But HRW researcher Lama Fakih, who spent several days in Latakia Province in September and spoke to residents, soldiers, militiamen, doctors and officials, said she could not confirm if the FSA were present on August 4 when the atrocities took place.

Lama Fakih, the Syria and Lebanon researcher in HRW's Middle East and North Africa Division, told Reuters in reference to the Latakia operation by rebels: "Homes were destroyed and burned. Most villagers had not returned."

Fakih met Hassan Shebli, an Alawite man from the village of Barouda, who fled his village at 4:30 a.m. on August 4 as rebels approached. He left his wife, who was in her 60s and needed canes to walk, and his son, 23, who was paralyzed, Fakih said.

Shebli said that they were both killed and buried behind his house. Fakih visited the house and saw bullet holes in the son's bed frame. "I was able to see the blood splattered on the wall," she said, showing a picture of the room.

Rebel footage posted on the Internet showed images of Shebli's son and wife with rebel terrorists during the operation.

The scale and organization of the attacks on civilians suggest premeditation and make them a crime against humanity, HRW said, rather than isolated war crimes reported during the Syrian war. The United Nations says the conflict has killed more than 100,000 people in two-and-a-half years.

"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."

Residents who returned to the villages said they found bodies of residents on the streets, in their homes as well as in piles of burnt corpses and in mass graves, according to Fakih.

The HRW report documented the involvement of over 20 rebel groups in the Latakia offensive, which started on August 4 and ended on August 18, when the government regained control of the area.

Syria has been gripped by deadly unrest since 2011. According to reports, the Western powers and their regional allies -- especially Qatar, Saudi Arabia, and Turkey -- are supporting the militants operating inside Syria.

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