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Extremist groups within the Syrian opposition are responsible for the mass killing of civilians, executions and hostage-taking in the countryside of north-western Syria, Human Rights Watch claims in a report released on Friday.

The rights group says an investigation into a military offensive by the rebels found strong evidence of war crimes and crimes against humanity.

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The offensive began in rural Latakia on 4 August -- the first day of the Muslim Eid al-Fitr holiday which marks the end of Ramadan – and lasted for two weeks. By the time it had ended, at least 190 civilians were killed and over 200 hostages taken, according to HRW. The rights group said at least 67 of the victims were executed or unlawfully killed in the attack on pro-government villages.

“These abuses were not the actions of rogue fighters,” said Joe Stork, acting Middle East director at Human Rights Watch. “This operation was a coordinated, planned attack on the civilian population in these Alawite villages.”

One survivor of the attack – a resident of a small hamlet between the villages of Blouta and al-Hamboushieh – described how fighters entered his neighbourhood and began firing indiscriminately.

“My mom was here in the house with me. She came out of the house first, and I was behind her. We saw the three fighters just in front of us, and then we fled on foot down behind the house and into the valley. The three fighters that I saw were all dressed in black. They were shooting at us from two different directions. They had machine guns and were using snipers. My older brother came down and hid with us as well. We hid, but my dad stayed in the house. He was killed in his bed. My aunt, she is an 80 year old blind woman, was also killed in her room. Her name is Nassiba.”

A member of the Darwish family from al-Hamboushieh told HRW about eight of his relatives who were killed during the opposition offensive.

“I heard about the news of the attack around 5:45 am. I got a call that there was a problem in our village. I called and tried to speak to all my brothers, but no one answered. I called Fatima Darwish, my sister, who answered the phone screaming “help me, rescue me, I’m injured.” She was on the roof of the house and told me that the fighters were killing and were slaughtering. I could hear Takbeer in the background and the screaming of children. I told her to run but she said the whole village was surrounded, and that people were being executed. She said that Waheeb Mariam [our neighbor] was executed in front of the house… Then the line cut.”

The resident was outside of the village when the attack took place, but returned 15 days later when the Syrian army had retaken control. At his sister’s house, he found her corpse and the bodies of three other relatives.

“I saw Yazen [my brother], Fatima, my niece Reem, and my mother. Fatima was in her second year at university. Reem…was visiting the village. My mom [Mona Salah Shibli] was next to them. Their bodies had been shot. There were many shots in them… over the whole body. I saw maybe 300 BKC shells [from a machine gun] in addition to lots of bullet shells on the roof.”

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The attack is the latest in a series of incidents that demonstrate the increasingly sectarian nature of the conflict in Syria, more than two and a half years since the war began.

The coastal region of Latakia is home to the majority of Syria’s minority Alawites, the minority sect to which President Bashar al-Assad belongs. Extremist groups within the largely Sunni opposition have been known to target Alawite and Christian areas in retaliation for government attacks against rebel-held territory.

Among the groups involved in the offensive was al-Qa’ida offshoot the Islamic State of Iraq and as-Sham (ISIS) and its affiliate Jabhat al-Nusra. Other Islamist groups such as Ahrar al-Sham, Jaish al-Muhajireen wal-Ansar and Suquor al-Izz were also involved in the operation.

In recent months, clashes between extremist rebels and more moderate groups within the opposition have been on the rise. ISIS in particular has sought to solidify control of areas they operate in.

The HRW report said that at least 20 distinct armed opposition groups participated in the operation they alternately termed the “campaign of the descendants of Aisha, the mother of believers”, the “Barouda offensive”, or the “operation to liberate the coast”.

Human Rights Watch has previously documented war crimes and crimes against humanity by Syrian government and pro-government forces. These include systematic torture and summary and extrajudicial executions after ground operations.

One the most severe crimes carried out by government forces was the killing of 250 people in the mostly Sunni towns of Banias and Bayda in May. The Syrian army has also carried out indiscriminate air strikes on civilian areas throughout the conflict.

The HRW investigation involved interviews with residents, survivors, emergency response staff, and fighters and activists on both government and opposition sides.

The report comes as chemical weapons inspectors said they had visited three sites where the weapons are stored across the country, 10 days into their disarmament mission.

Operating on a rare consensus, the UN has tasked the Organisation for the Prohibition of Chemical Weapons to rid Syria of its stockpile by mid-2014. The UN hopes to organise cease-fires between rebels and government forces to ensure safe passage.