Rebel opposition forces in Syria have killed at least 190 civilians and seized dozens of child hostages who they have paraded on a YouTube video, Human Rights Watch says.
Fighters burned villages, threw bodies in mass graves and kidnapped women and children in acts described as "crimes against humanity" by HRW.
Witnesses described 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.
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 Lattakia 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.
"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.
The rebels targeted Alawite villages where most inhabitants were considered loyal to Syria's President 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.
The news comes 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.
The Norwegian Nobel Committee handed the award to OPCW "for its extensive efforts to eliminate chemical weapons."
"The conventions and the work of the OPCW have defined the use of chemical weapons as a taboo under international law," the committee said.
"Recent events in Syria, where chemical weapons have again been put to use, have underlined the need to enhance the efforts to do away with such weapons."