At least 2,015 civilians, including 281 children and 138 women, were killed last month, while Free Syrian Army (FSA), rebels and other allied anti-government groups, lost 1,448 fighters, the London-based group, which has a large network of activists on the ground across Syria, said.
A total of 700 people belonging to the Shuaitat tribe, the majority of them civilians, were executed last month in the eastern province of Deir al-Zur by radical militants who style themselves fighters of the Islamic State (IS) Sunni militant organisation.
Militant groups aligned with the IS or the Al Nusra Front (an affiliate of Al Qaeda in Syria) lost 1,351 fighters, including many foreign volunteers.
Forces loyal to President Bashar al-Assad's regime lost at least 1,405 soldiers and 817 militia fighters.
The number of victims may rise to 8,000 because verifying the casualty count on both sides is difficult, the human rights group said.
Syria has been gripped by a deadly conflict since March 2011 displacing millions of people, many of them taking refuge in neighbouring Lebanon.