Abdulrahman, who tracks the conflict using sources on the ground, said it was the biggest single massacre of civilians by Islamic State since the killing of hundreds of members of the Sunni Muslim tribe Sheitaat tribe in eastern Syria last year.
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The Islamic State group launched the two-pronged offensive in northern Syria on Thursday after several setbacks, re-entering the symbolic battleground town of Kobane and seizing parts of the city of Hasakeh.
In southern Syria, an alliance of rebel groups, including Al-Qaeda affiliate Al-Nusra Front, also attacked government-held areas of the city of Daraa in another potentially serious blow to the regime.
Analysts said the surprise IS assaults were aimed at diverting Kurdish forces after they scored a series of victories and advanced on the militants' Syrian stronghold of Raqa.
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Kobane, on the border with Turkey, became an important symbol in the battle against IS after the group launched a bid to take it last year.
Kurdish forces backed by US-led air strikes waged a four-month battle to repel the group, finally securing the town in January.
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