Syrian rebel forces killed as many as 190 civilians and seized more than 200 hostages during a military offensive in August, Human Rights Watch says.
In a report, HRW says the deaths occurred in pro-government Alawite villages in rural Latakia governorate.
It said the findings “strongly suggest” crimes against humanity were committed.
Rebels and government forces have both been accused of abuses since the uprising against President Bashar al-Assad began more than two years ago.
Syrian opposition forces comprise many groups, some of which are allied to al-Qaeda.
HRW says it conducted an on-site investigation and interviewed more than 35 people, including survivors and fighters from both sides of the offensive.
In its 105-page report, it says that in the early hours of 4 August opposition fighters overran government positions in the Latakia countryside of northern Syria and occupied more than 10 Alawite villages.
This operation was a co-ordinated, planned attack on the civilian population in these Alawite villages”
Joe StorkHuman Rights Watch
The New York-based rights group says it appears the civilians were killed on the first day of the operation.
Women and children
“Witnesses described how opposition forces executed residents and opened fire on civilians, sometimes killing or attempting to kill entire families who were either in their homes unarmed or fleeing from the attack,” the report said.
HRW says about 20 opposition groups took part in the offensive and that five were involved in the attacks on civilians. Those five do not include the main rebel forces, the Free Syrian Army.
The report says two groups – the Islamic State of Iraq and Sham and Jaish al-Muhajireen wal-Ansar – are still holding the hostages, most of them women and children.
The government launched a counter-attack the next day and regained control of the area on 18 August.
Joe Stork, acting Middle East director at HRW, said the abuses were “not the actions of rogue fighters”.
“This operation was a co-ordinated, planned attack on the civilian population in these Alawite villages,” he said.
HRW says evidence including witness statements and a review of hospital records showed opposition forces executed or unlawfully killed at least 67 of the 190 dead civilians who were identified.
It says the high civilian death toll and the nature of the recorded wounds “indicate that opposition forces either intentionally or indiscriminately killed most of the remaining victims”.
“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,” the report said.
HRW says Islamist rebel groups – which include foreign fighters – are financed by individuals in Kuwait and the Gulf. It calls on the UN to impose an arms embargo on all groups credibly accused of war crimes.
Earlier this year, the UN High Commissioner for Human Rights, Navi Pillay, called on the UN Security Council to refer Syria to the International Criminal Court (ICC).
She said it “would send a clear message to both the government and the opposition that there will be consequences for their actions”.
Some 100,000 people have been killed in the conflict.