Jihadists in Syria were accused Friday of war crimes over the killing of 190 civilians from President Bashar al-Assad's Alawite minority, in the largest atrocity attributed to rebel fighters.
Human Rights Watch said another 200 people -- the vast majority women and children -- were taken hostage in the operations that took place in August and are still being carried out.
The report, which urged an arms embargo on groups suspected of war crimes or crimes against humanity, said at least 67 of the victims were "executed or unlawfully killed".
It comes as NATO chief Anders Fogh Rasmussen said he sees no military solution to the 31-month conflict in Syria, which has killed more than 115,000 people.
HRW said the killings began on August 4, the first day of the Eid Al-Fitr holiday ending the Muslim fasting month of Ramadan, in a sweep of villages in Latakia province, an Alawite stronghold.
"These abuses were not the actions of rogue fighters," said HRW's Joe Stork. "This operation was a coordinated, planned attack on the civilian population."
The 105-page report, based on interviews with 35 survivors, emergency personnel and fighters on both sides, said at least 20 groups were involved, but that five "are responsible for specific incidents that amount to war crimes".
It named them as Ahrar al-Sham, Islamic State of Iraq and the Levant (ISIL), Al-Nusra Front, Jaish al-Muhajireen Wal-Ansar and Suqur al-Ezz.
HRW said that, in some cases, opposition fighters executed or gunned down entire families, or killed the elderly or infirm who had been left behind by those who fled.
HRW also said "some of the opposition atrocities... had clear sectarian motivation".
In one village, it said fighters intentionally damaged an Alawite maqam, a site where a religious figure is buried, and "appear to have intentionally damaged and dug up the grave".
It said they had also abducted and executed Sheikh Bader Ghazzal, the area's Alawite religious leader, quoting Al-Nusra as saying he had been executed because he supported the regime.
Meanwhile, HRW quoted opposition forces, including an officer involved in negotiations, as saying ISIL and Jaish al-Muhajireen Wal-Ansar are holding 200 hostages, mostly women and children.
It called for them to be treated humanely and released immediately, urging countries with influence over the groups to press for their liberation.
It quoted sources as saying funding for the operation had come "from individuals in Kuwait and other Gulf countries". It also said Turkey should do more to "prevent the entry of fighters and arms" to groups implicated in "systematic human rights violations".
"We have documented abuses by opposition forces before, but never on this scale," HRW researcher Lama Fakih told AFP.
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"Violators on all sides should be sent a message by the UN Security Council" which should refer the situation in Syria to the International Criminal Court.
The opposition National Coalition expressed "deep concern" about the reported atrocity and distanced itself from those who perpetrated it.
"The incident reported by HRW in today's report does not represent an effort by the true Syrian opposition, but rather a shameful one-time attack by outlier extremist groups that thrive under the hand of the Assad regime," it said in a statement.
Russia, meanwhile, accused Syrian rebels of receiving chemical weapons training in lawless parts of Afghanistan and planning attacks on the territory of Iraq.
In Athens, NATO's Rasmussen "there is no military solution to the conflict in Syria", emphasising a political solution was required to end it.
On the ground, a second team of inspectors from the Organisation for the Prohibition of Chemical Weapons arrived in Damascus on Thursday to help supervise the destruction of Syria's chemical arsenal under the terms of a UN resolution.
At the same time, the Security Council backed a plan by UN leader Ban Ki-moon for a joint mission of up to 100 experts with the OPCW to destroy the weapons, diplomats said.
The council agreed that the UN and the OPCW were doing "a great job," said France's UN ambassador Gerard Araud, while Russian envoy Vitaly Churkin noted "good cooperation by the Syrian government".
The OPCW was on Friday named this year's winner of the Nobel Peace Prize, in what French President Francois Hollande called a "vindication" of its work in Syria.
Shortly after the announcement, a six-vehicle OPCW convoy left its Damascus hotel on its first mission since the announcement of the award.