GAZIANTEP, Turkey — Syrian Islamist rebel groups Friday were accused of killing at least 190 villagers in the country’s pro-government heartland and kidnapping hundreds more, as the emergence of jihadist forces fuels further sectarian strife.

Civilian residents, including dozens of women and children, were gunned down or stabbed Aug. 4 during a coordinated, planned attack on villages in the mountains of Latakia province in August, New York-based Human Rights Watch said. More than 200 residents taken hostage remain in the custody of Islamist rebels, the group added.

The offensive, led by al-Qaida’s Islamic State of Iraq and Sham (ISIS), targeted villages largely home to Alawites, members of the Shiite offshoot sect to which President Bashar Assad belongs.

“This is the first time that we have documented opposition forces actually systematically targeting civilians,” said Lama Fakih, the group’s deputy director in Beirut, who last month visited five of the villages. She also reviewed medical records and interviewed 19 witnesses, as well as doctors, military officials and opposition members for the report.

The number and methodical nature of the killings constituted a “crime against humanity,” Fakih said.

A video of the Human Rights Watch’s visit showed one man fleeing the attack and leaving his wife and paralyzed son at home.

The report called for an arms embargo against rebel forces responsible for funding, arming and planning the operation, including ISIS, Ahrar al-Sham, Jabhat al-Nusra, Jaish al-Muhajireen wal-Ansar and Suquor al-Izz.

Material from The New York Times is included in this report.