Rebel forces in Syria have killed at least 190 civilians and seized dozens of child hostages, who they have paraded in a video posted on YouTube, Human Rights Watch says.

Fighters burned villages, threw bodies in mass graves and kidnapped women and children in acts described as likely "crimes against humanity" by HRW.

Witnesses gave harrowing accounts of family members being executed in their beds during an attack on August 4.

One man said he was forced to flee, leaving his paralysed son and wife to die at the hands of armed rebels.

Graves in a back yard of a village home. Pic: Human Rights Watch

A 105-page report by the New York based human rights group said the findings "strongly suggest" crimes against humanity were committed in the rural Latakia area.

HRW said it conducted an on-scene investigation and interviewed more than 35 people, including survivors and fighters from both sides.

One child said: "My mum was here in the house with me. She came out of the house first, and I was behind her.

"We saw the three fighters just in front of us, and then we fled on foot down behind the house and into the valley.

"The three fighters that I saw were all dressed in black. They were shooting at us from two different directions. They had machine guns and were using snipers.

Picture of body bags released by the Ministry of Health

"My older brother came down and hid with us as well. We hid, but my dad stayed in the house. He was killed in his bed.

"My aunt, she is an 80-year-old blind woman, was also killed in her room. Her name is Nassiba."

HRW said two opposition groups - the Islamic State of Iraq and Sham and Jaish al-Muhajireen wal-Ansar - were holding around 200 hostages from Alawite villages, where most inhabitants were considered loyal to Syria's leader Bashar al Assad.

"The evidence strongly suggests that the killings, hostage-taking, and other abuses committed by opposition forces on and after August 4 rise to the level of crimes against humanity," Human Rights Watch said.

Rebel fighters attacking villages. Pic: Human Rights Watch

The report came the Organisation for the Prohibition of Chemical Weapons (OPCW) continued its mission to deal with Syria's chemical weapons stockpile.

International inspectors have so far visited three sites linked to Syria's chemical weapons programme, OPCW spokesman Michael Luhan said.

In another development OPCW, based in The Hague, was awarded this year's Nobel Peace Prize.