More the 230 bodies have been found in a mass grave in Syria’s eastern Deir al-Zor province, a group monitoring conflict in the country said on Wednesday.
Members of the extremist group Isis have not claimed responsibility for the deaths, but the dead are believed to be members of the al-Sheitaat tribe which had battled the militants, the Britain-based Syrian Observatory for Human Rights said.
The tribe is from Deir al-Zor province and numbers about 70,000. If confirmed, their deaths would bring the number of Sheitaat members killed by the Isis to over 900.
Omar Abu Layla, a spokesman for the moderate rebel umbrella Free Syrian Army group in Syria's east, told Reuters that Sheitaat tribespeople had discovered the mass grave as they returned to their homes. Isis, which calls itself the Islamic State, is occupying the area and had given them permission to return.
"This is a message from Daesh that if there is any attempt at revenge, your fate will be the same as your relatives," he said, using a derogatory Arabic acronym for the Islamic State group.
Over the course of this year, Isis has seized swathes of Syria and Iraq, and currently controls all but a few pockets of Deir al-Zor province.
In pictures: Syria conflict
In pictures: Syria conflict
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Syrians carry children amid debris following a air strike by government forces in the northern city of Aleppo
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A Syrian man carries a girl on a street covered with dust following a air strike by government forces in the northern city of Aleppo
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Syrians react as they stand amid debris following a air strike by government forces in the northern city of Aleppo
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A Syrian man carries a girl amid debris following a air strike by government forces in the northern city of Aleppo
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An injured Syrian man walks out from the rubble of a destroyed building following a air strike by government forces in the northern city of Aleppo
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A Syrian woman makes her way through debris following a air strike by government forces in the northern city of Aleppo
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People stand on the rubble of collapsed buildings at a site hit by what activists said was a barrel bomb dropped by forces loyal to Syria's President Bashar al-Assad, in the Al-Fardous neighbourhood of Aleppo
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Syrian residents stand amid the rubble of destroyed buildings
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A Syrian resident grasps a mattress amid rubble in the al-Firdous neighborhood of the northern city of Aleppo
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A bullet-riddled parking sign stands amid debris in a deserted street leading into the old city of Homs
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A general view shows abandoned buildings on a deserted square in the old city of Homs after Syrian government forces regained control of rebel-controlled areas
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A mosque is pictured through shattered glass in the old city of Homs, as rebel fighters withdrew from the city centre in line with a negotiated withdrawal deal with the government after having held out under tight siege for nearly two years
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Buses carrying Free Syrian Army fighters leaving Homs. Exhausted and worn out from a year-long siege, hundreds of Syrian rebels left their last remaining bastions in the heart of the central city of Homs under a cease-fire deal with government forces. The exit of some 1,200 fighters and civilians will mark a de facto end of the rebellion in the battered city, which was one of the first places to rise up against President Bashar Assad's rule, earning it the nickname of "capital of the revolution"
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Syrian government forces hold up a portrait of President Bashar al-Assad (L) while others raise the national flag on top of a pole in the old city of Homs
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Forces loyal to Syria's President Bashar al-Assad run through Aleppo's Bustan al-Qasr crossing after their release by rebels. They were freed as part of a larger deal which saw the last remaining Syrian rebels in central Homs city evacuate their positions and free captives in several locations in northern Syria
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A Syrian woman and two children walk past heavily damaged buildings in the northern city of Aleppo
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A man carries a wounded girl following a reported bombardment with explosive-packed "barrel bombs" by Syrian government forces in the al-Mowasalat neighborhood of the northern Syrian city of Aleppo
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A wounded man sits as he is treated at a makeshift hospital following a reported bombardment with explosive-packed "barrel bombs" by Syrian government forces in the al-Sakhour district of the northern city of Aleppo
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Debris rises in what Free Syrian Army fighters and Islamic rebels said was an operation to strike Al-Sahaba checkpoint, which is considered a gateway to Al-Dayf valley, and remove forces loyal to Syria's President Bashar Al-Assad in Maarat Al-Nouman, Idlib province
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Men try to put out fire at a site hit by what activists said was an air strike by forces loyal to Syria's President Bashar al-Assad in the town of Azaz, north of Aleppo, near the border with Turkey
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Civil Defence members try to put out fire
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Survivors react at a site hit by what activists said was an air strike by forces loyal to Syria's President Bashar al-Assad in the town of Azaz, north of Aleppo, near the border with Turkey
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Residents queue as they wait to receive food aid distributed by the UNRWA at the Palestinian refugee camp of Yarmouk, south of Damascus
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Belongings of Syrian rebels inside a chapel at Crac des Chevaliers, the world's best preserved medieval Crusader castle in Syria. The village was destroyed in fighting between the government and rebel forces while the castle, listed as a UNESCO world heritage site, also has been damaged over the past two years
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Hosen Sabah, a 16-year-old student is comforted by his mother at a hospital in Damascus. Nosen was wounded by a mortar outside his school, while 14 other students were killed and over 80 wounded
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A Free Syrian Army fighter works on a locally made launcher before firing it towards forces loyal to Syria's President Bashar Al-Assad in Mork town
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Syrian policemen and citizens inspecting the site of a car bomb at the entrance of Moadhamiyet al-Sham neighborhood in rural Damascus. According to Syria's Arab News Agency (SANA), a car bomb explosion has gone off in the countryside of Damascus and initial information say there are casualties, where a car rigged with explosions was remotely detonated at the entrance of Moadhamiyet al-Sham neighborhood in rural Damascus during engineering units it was trying to dismantled it
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Opposition fighters carrying a rocket launcher during clashes against government forces in the Sheikh Lutfi area, west of the airport in the northern Syrian city of Aleppo
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A Syrian man helps a woman to make her way through debris following reported air strikes by government forces in the Halak neighbourhood in northeastern Aleppo
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A Syrian man reacts as he carries the body of injured boy following reported air strikes by government forces in the Halak neighbourhood in northeastern Aleppo. According to the Syrian Observatory for Human Rights, at least 33 civilians were killed in the attack
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Syrian rescue workers carry the body of a woman following reported air strikes by government forces in the Halak neighbourhood in northeastern Aleppo
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Syrians gather at the site of reported air strikes by government forces in the Halak neighbourhood in northeastern Aleppo
The group has used revenue from the province’s oilfields to fund its operations, but faces pressure since a US-led coalition started launching air strikes against it in Syria in September.
The gruesome discovery comes after the militant group had killed some 700 members of the Sheitaat tribe in August - the majority of them civilians - over the preceding two weeks after conflict flared when the militants took over two oilfields.
The Observatory, which has tracked violence on all sides of the nearly four-year-old conflict, said beheadings were used to kill many of the tribe's members.
Isis fighters are currently battling Syrian President Bashar al-Assad's forces near Deir al-Zor city for a military air base that is one of the government's last strongholds in the country's east.
The Britain-based Syrian Observatory for Human Rights also revealed today that more than 120,000 fighters supporting Syrian President Bashar al-Assad have been killed in the country's civil war since it began in 2011.
In 2011, Syria's conflict began as a peaceful protest movement calling for reforms in quickly but descended into civil war after a government crackdown.
In total, more than 200,000 people have been killed and millions more have fled their homes.
The unstable situation means exact death tolls are difficult to verify, but the figures calculated by the Observatory are widely regarded as credible. The United Nations estimated in August more than 190,000 people had died in the conflict.
Additional reporting by ReutersReuse content