While it is clear that the vast majority of war crimes and other gross violations continue to be committed by government forces, ...
In a recent briefing, Summary Killings and other abuses by armed opposition groups, the organization documented dozens of such cases.
Amnesty International has most recently documented violations by Syrian security forces and pro-government militias in the briefing, Government bombs rain on civilians, in a long line of reports highlighting targeting of civilians, arbitrary arrest, torture and other ill-treatment, enforced disappearances, extrajudicial executions and other abuses since the start of the Syrian uprising in March 2011.
http://www.amnesty.org/en/news/keep-international-pressure-against-abuses-syria-2013-03-26
At least eight massacres have been perpetrated in Syria by President Bashar Assad's regime and supporters and one by rebels over the past year and a half, a U.N. Commission of Inquiry on Syria said Wednesday.
The commission's probe highlights the worsening pattern of violence against civilians, including executions and hospital bombings, as the government battles to retake lost territory from the rebels, including Islamist foreign fighters who also have carried out war crimes.
http://abcnews.go.com/International/wireStory/probe-massacres-syria-regime-rebels-20221533
Amnesty International termed it "the vast majority" of war crimes have been perpetrated by government forces. The UN Commission of Inquiry on Syria found an 8-1 ratio of "massacres" committed by government vs. the rebels.
If you have contradictory information, I would love to see it.