Story highlights Monitoring group says Syria mass killing is 'the largest execution operation' this year

In the Iraq attack, ISIS gunmen opened fire before blowing themselves up

(CNN) ISIS killed 33 people execution-style in eastern Syria on Wednesday, according to a monitoring group.

The Syrian Observatory for Human Rights reported that the terror organization carried out the mass killing in the the al-Mayadin desert near the strategic city of Deir Ezzor on Wednesday morning, it said, adding that its activists were "able to monitor" the incident.

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The London-based monitoring group called it "the largest execution operation carried out by the Islamic State organization in 2017."

The report said the people were between ages 18 and 25 and were "killed by sharp tools." It added that it is unknown whether the victims were Syrian government forces, allied militia or rebel factions.

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