The killing of members of the Shueitat tribe come as Islamic State group fighters close in on the last government-held army base in the region.
Syrian warplanes bombed the extremists' positions today in an attempt to halt their advance.
A Turkey-based activist who is originally from Deir el-Zour and is in contact with people in the province told The Associated Press that the Islamic State killed as many as 200 members of the Shueitat tribe.
The activist spoke on condition of anonymity for fear that family members in Syria might be harmed.
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Journalists do not have access to the far reaches of eastern Syria under Islamic State control, and it was not immediately possible to verify the death toll. But differences in casualty figures are common in the chaos of Syria's civil war.
Last week, Islamic State fighters crushed the Shueitat tribal uprising against their rule in eastern Syria after three days of clashes near the border with Iraq.
"They considered all members of the Shueitat tribe apostates because they rose against them," the activist said. "Some men were taken out in the fields and beheaded them while others were shot in the head."
The Islamic State group has declared a self-styled caliphate in territory it controls straddling the Iraq-Syria border, imposing a harsh interpretation of Islamic law.
They have killed hundreds of people in both countries over the past two months, including some who were beheaded and their heads were later displayed in a public square in the northern Syrian city of Raqqa.
They now are closing in on the Tabqa air base, the last position held by Syrian government troops in Raqqa province. The Observatory reported intense clashes Monday between troops and Islamic State fighters on the edge of the villages of Ajil and Khazna near the Tabqa air base. It said there were casualties on both sides.