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.
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, the BBC reports. It says the findings "strongly suggest" crimes against humanity were committed.
Rebels and Syrian government forces have both been accused of abuses. 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.
Entire families killed
HRW says it appears the civilians were killed on the first day of the operation, the BBC reports.
"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 - 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.
HRW acting Middle East director Joe Stork 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.
More than 100,000 people have been killed in the conflict, according to the UN.
Payment cards for refugees
Meanwhile, the United Nations World Food Programme says it will distribute payment cards to Syrian refugees in Lebanon as a way of making sure they get the supplies they need.
The cards will be loaded with about $US27 per person per month and can be used to buy goods from a list of items in hundreds of shops near the Syrian border, helping to boost the local economy, the BBC reports.
This means the refugees can buy fresh produce, such as milk and cheese, which is not normally included in traditional food rations. Greg Barrow, from the World Food Programme, says the project is expected to give the cards to 800,000 people before the end of the year.