Rebel forces in Syria killed as many as 190 civilians and seized more than 200 hostages during a military offensive in August, Human Rights Watch says.
It said the findings "strongly suggest" crimes against humanity were committed.
A report by the New York-based group says the deaths occurred in villages inhabited predominantly by members of President Bashar al-Assad|s minority Alawite sect near the city of Latakia.
Syrian opposition forces comprise many groups, some of which are allied to al-Qaeda.
Human Rights Watch (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 and occupied more than 10 Alawite villages.
HRW says it appears the civilians were killed on the first day of the operation.
HRW says about 20 opposition groups took part in the offensive and that five were involved in the attacks on civilians - the al-Nusra Front, the Islamic State of Iraq and the Levant (ISIS), Jaysh al-Muhajirin wa al-Ansar, Ahrar al-Sham and Suqour al-Izz. None are affiliated to the Western-backed Supreme Military Council of the Free Syrian Army.
The report says ISIS and Jaysh al-Muhajirin were 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.
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.
Human rights organisations also accuse Syrian government forces of killing civilians during the country|s 31-month long conflict, most recently in a poison gas attack near Damascus on 21 August.
Western nations blamed government forces for the attack, in which hundreds died, but President Bashar al-Assad says rebel fighters are responsible.
The attack led to a disarmament deal in which Syria agreed to allow its chemical weapons arsenal to be destroyed, placing on hold the prospect of US-led strikes on Syria as punishment for the Damascus attack.
Experts from the global chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons (OPCW), are currently overseeing efforts to destroy weapons production equipment in Syria.
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".
More than 100,000 people have been killed in the conflict, according to the UN.
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