"The Islamic State (IS) group has executed 16 men in Deir Ezzor and one more in Raqa, to send a message to all their opponents after recent assassinations of 12 Syrian, Iraqi and Algerian jihadists," said Syrian Observatory for Human Rights director Rami Abdel Rahman.
"The men were accused of fighting IS," Abdel Rahman told AFP, adding that only one of them had been proven to be linked to the assassinations.
The group emerged in Syria's war in 2013 and in June declared a "caliphate" in areas under its control straddling Syria and Iraq.
It has committed some of the war's worst abuses, carrying out near-daily executions in areas in its grip.
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Most of Raqa and Deir Ezzor provinces are IS-controlled.
Among those executed in the past two days in oil-rich Deir Ezzor were five members of the Shaitat tribe, which launched a local rebellion last year against IS.
Syria's war has killed more than 200,000 people in nearly four years.