The so-called Syrian Observatory for Human Rights documented in the report that at least 130,000 people have also been forced out of their homes since then.
The UK-based group said Takfiri militants of the al-Qaeda-linked Islamic State of Iraq and the Levant (ISIL) were preventing food and medical supplies from reaching some neighborhoods of Deir al-Zour in eastern Syria.
The infighting has escalated over the past months amid gains by the Syrian army across the country.